Here's an updated version, incorporating a simple power-to-mass scheme.

Formatting is wacky in places, and I don't know why. Final output will be PDF, so it's not a big issue.  

You'll see that I'm ignoring weight-based encumbrance, looking at power and mass effects (general "slowness") only. It's analogous to introducing only MSR rules from the old GVER.

True encumbrance really complicates the picture. It needs to be plugged into the encumbrance rules, with a cost worked out for quirky miscellaneous effects (such as increased fatigue, and a permanent level of encumbrance that affects remaining carrying capacity). Also, the interaction of mass-based slowness and weight-based encumbrance, while interesting, is complex. And then you've got gravity and buoyancy as side topics...

It's much easier for this text's purpose – and for most players – to assume that a creature handles its own weight, and look at just the effect of power and mass instead. That's handled by simple adjustments to DX, Basic Speed, and Move.

A full set of rules in the future may want to tackle explicit encumbrance as well, but I think the simple approach here does a fine job for most critter needs, and goes down smoothly.

Under optional extras for size, I modified the simple DX suggestion, and added a Basic Speed suggestion. These mesh with the later more detailed suggestions. I similarly used a simple new approach to Move: no more multipliers, just a real rough suggestion for what expected Move might be. (It cleverly uses 4e's SM Table, the kind of nifty trick you can do with log progressions.)

Everything falls under the "suggested stats" approach, rather than strict formulae. 4e hews to that approach more than 3e did, and I'm happy to go along. Overall, I think v0.2 is neat and clean. How about you?

GULLIVER Mini draft v0.2

(page references: B = Basic Set, BT = Bio-Tech, F = Fantasy. All editions 4e.)

Building Odd-sized Characters

The Basics 

GURPS 4e offers the tools you need to build odd-sized characters, with
some details outlined in BT62-64 and F50-51. But the Basic Set doesn't
explicitly detail the process. Here's your simple guide to the basics:

1. Choose your height. Note your height multiple: the multiple of your height to the human-normal 2 yards.
2. Choose your SM based on your height (B19).
3. Choose your starting ST, HP (usually the same as ST), DR, and weight – those appropriate for a SM 0 (human-sized) version of the design.
4. Multiply starting ST, HP, and DR by height multiple. Multiply
starting weight by the cube of height multiple. (Quick cheat:
Multiply/divide weight by 10 for every full two SM levels above/below
0, and then by 3 for a remaining odd level above/below.)
5. Add optional extras (below) if desired. Pay for final stats normally. (Don't forget -10% cost of ST and HP per SM above 0, max -80%.)

SM itself has no cost, and includes the following changes:

Reach: For literal reach, multiply 1 yard by height multiple. For combat Reach, see B402.
Sustenance requirements: Multiply air, water, and food requirements by the square of height multiple. (See BT63 for detail on meal size and frequency.)

Non-humanoids

The above rules are for humanoids. For most other creatures, replace
height with length. Measure roughly from head to (hind leg) toe,
ignoring a long neck or tail.

A round or boxy shape modifies SM for TH purposes only (B19), not for
stat modifications as described here.

The hex size for a horizontal creature is roughly the square of height multiple. Halve hex size for humanoids. Round hex size up to 1 for all but tiny creatures.      

Working backward

You can start with an odd-sized design (such as a real animal),
instead of starting with a SM 0 version as above. Estimate SM from
known height/length or weight. Estimate ST and HP as either a) some
reasonable starting ST and HP, times height/length multiple, or b)
twice the cube root of weight. Adjust ST and HP from there as you
like. 

Optional extras: DX, Basic Speed, and Move

Size greatly affects the ratio of power to mass, and thus agility, of a typical creature. GURPS handles power and mass on B19 with a short suggestion to modify Basic
Move as you like. That's fine, but it's easy to eyeball more specific size-based adjustments to the "agility stats": DX, Basic Speed (and Dodge!), and Move. Use the below as suggestions only; modify as you like, and ignore if you don't want them, or if your design already reflects them well enough.

DX: Halve your SM (round down). Subtract that amount from DX. (Positive SM lowers DX; negative SM raises it.)

Basic Speed: Halve your SM (round up). Subtract that amount x 0.25 from Basic Speed. (See B17. The above DX mod will also affect Basic Speed.) Don't reduce Basic Speed below 1.

Move: Add SM +
the above DX mod + 2. Find that under the SM column of the SM Table
(B19). The given yards
roughly suggest expected
Move for the typical power and mass of an odd-sized creature, before any other special
considerations (like four legs, posture, fitness, etc.).

If current Move is far from the suggestion, consider adding or
subtracting Basic Move (B17) to approach it. (You can use B52's
Enhanced Move as well, though its limitations more realistically
reflect adaptations such as four legs, not size and power.) But don't feel bound by the suggestion; especially in non-humanoids and supers, Move can be just about whatever you want!

Net effects: Net +1 DX and +0.5 Basic Speed per two levels of SM
below 0, the reverse effect for SM above 0. Move will scale with (roughly) the square root of height multiple.

Optional extras: other 

The below are purely optional add-ons. Purchase normally.

Overall ST: +5% ST per SM above 0, for the bulky builds of large creatures.
Lifting ST: Many modifications aid with bearing
weight, but not other uses of ST. Buy extra ST as additional Lifting ST
(B65), for each of the below:

SM: +10% per full or partial 3 levels above SM 0, for structural changes including thick bones.
Load-bearing design: +10% (or more for magic and high-tech). Good for a Dwarf or draft animal.
Legs: +10% for 3 to 5 legs, +20% for 6 to 7 legs, +30% for 8+ legs or no legs.
Splay-legged stance: -10% or worse.

Option: For more realism, change the above Lifting ST boost into a new trait, Carrying ST. This adds to ST only for carrying, pushing and pulling, not to lifting or to applying slow pressure. Cost: 2 points / +1.
IQ: No change by default, but BT63 has suggestions for small creatures.
HT: No change by default, but consider -1 HT per full
three levels of SM above 0, for the difficulty of nutrient transport,
waste removal, immune response, etc. in such a huge form.

Design example 

For a SM 4 creature, first set human-size starting stats. Height multiple is 5. Multiply starting ST, HP, DR, and Reach by 5, sustenance requirements by 5x5 = 25, and starting weight by 5x5x5 = 125.

For agility mods, consider -2 DX and -0.5 Basic Speed (total -1 Basic Speed after the -2 DX). As a Move suggestion, look up SM 4 -2 for DX mod + 2 = SM 4: that suggests Move 10, barring other considerations.

For other mods, consider an extra 20% ST and extra 10% Lifting (or Carrying) ST for build, and -1 HT. Buy all stats
normally (with -40% on all ST).

Play guide

SM in melee combat

Use relative SM (the difference in attacker and target SM)
as a bonus on the smaller creature's melee TH and a penalty on the
larger creature's. Treat hit location TH modifiers as additional
relative SM modifiers. The final net TH modifier for SM may not exceed
+4.

Example: A SM 4 Giant hitting a SM -1 Dwarf takes a -5 on
melee TH. The Dwarf gains a +4 (not +5) TH vs the Giant. If the Dwarf
aims at the Giant's leg (-2 TH), the leg is essentially a SM 2 target;
the Dwarf gets a net +3 TH.

Low-ST damage

The ST damage chart doesn't work so well for small creatures. If you
prefer, use damage for ST 10 instead (thr 1d-2, sw 1d), and multiply
the damage roll by character ST/10. Round to the nearest.

This method is especially useful for ST 5 and under, but can be used for any ST under 10 if you like.  

Skill use

Where appropriate, apply a -1 penalty per difference in SM between a
character's SM and the ideal SM for the task. Examples include
controlling a mount or vehicle, picking a lock, pickpocketing, or
making armor for a small race. F137 suggests doubling the penalty when
using tools or weapons designed for a specific SM.

Power and Mass

The rules below replace the above suggestions for DX, Basic Speed, and Move with more specific suggestions based on actual power and mass, not just a generalization from size alone. In addition to plodding brontosauruses and zippy squirrels, they'll generate hyperkinetic SM 0 Spidermen as well.

These rules technically look at power and body mass alone, and don't consider encumbering weight; they assume that creatures can carry their own weight, and that ST measures power left over after accounting for excess weight. Any creature built with the rules on this page uses all normal encumbrance rules.

PMR (Power to Mass Ratio)

PMR Table (for additional levels, follow same progression)

PMR       Agility
…              …
0.01       -6
0.02       -5
0.05       -4
0.1       -3
0.2       -2
0.5       -1
1       0
2       1
5       2
10       3
20       4
50       5
100       6
…       …

(excuse formatting for the moment!)

Agility 

1) Start with BL (don't include Lifting or Carrying ST for this purpose). Take BL x 10 / naked body mass. That's your PMR.

2) Find your Agility on the PMR Table. (For PMR falling between two listed values, choose the smaller value – i.e., PMR 1.3 becomes PMR 1.)

Agility is nothing more than suggested mods to DX, Basic Speed, and Move, as follows:

DX: Buy DX equal to Agility.

Option: For more realism, buy limited DX for full-body, athletic actions only: no
effect on fine tasks as listed under High Manual Dexterity (B59) and no effect
on missile weapon TH. (When in doubt on whether or not to apply Agility,
halve it). The appropriate PMR is a prerequisite to buy DX (athletic only). Cost: ±15
per ±1. 

Basic Speed: Buy added Basic Speed equal to Agility x 0.25. Don't reduce Basic Speed below 1.

Move: Add SM + Agility + 2. Find that under the SM column of the SM Table (B19). The given yards suggest a ballpark realistic Move, before any other special considerations. Purchase as described earlier.

Net effects: Every rough doubling of PSR will add +1 DX and +0.25 Basic Speed (total 0.5 Basic Speed after +1 DX); every rough halving of PSR subtracts the same. Move will roughly scale with height multiple and the square root of PMR. (PMR typically scales with the inverse of height multiple, so Move typically scales with a net square root of height multiple.)

Design Example

A strong, lithe halfling PC has ST 6 and weighs 14 lbs. BL is
7.2. PMR = 7.2 x 10 / 14 = 5.1, which qualifies for Agility 2. That
suggests +2 DX, and additional +0.5
Basic Speed (total +1 after DX mod).

For suggested Move, he adds SM -2 + 2 for Agility + 2, for a net 2. The SM Table equates that with a suggested Move 5.

Looking at SM alone under the earlier rules suggests +1 DX, +0.5 Basic Speed, and a Move of about 3 for a halfling. But actual results can vary greatly under the detailed rules, as this example shows. Keep in mind that results are only suggestions – but if you follow them here, this particular hardcore halfling is ready to race the big folks! 

That's all

The above is a simple but fairly thorough guide to building both size and realistic power-to-mass considerations into your designs. Remember that all it offers is suggestions for ST, weight, Move, and so on. While you shouldn't feel bound by any of them, roughly following the suggestions will inject nicely consistent stats and performance capabilities into designs of any size.

Question: Does it make sense that I left out Lifting (or Carrying) ST from BL to determine PMR? Lifting ST would be vital if we were looking at resisting weight with power, but power-vs-mass agility would appear to stem from the "explosive" power that GURPS excludes from Lifting ST. Tough call, but I think I have it right. Discuss.

Question: Note that I'm fudging my cost for DX (athletic only). It can't be built exactly. -1 athletic DX seems to be -1 DX and +1 High Manual Dexterity, for -15 pts. But there's no corresponding "Low Manual Dexterity" to use in building +1 athletic DX, only the lesser-value Ham-Fisted; subtracting 5 from cost is too big a bargain. 

So I fudged, and tossed in "no missile weapon TH mod" to make a neat 15 pts seem better. Looks OK to me, but what do you readers think?