Ok, most of this should be review for you guys, but I did some tests and crunched together the numbers to figure out how stats are calculated.
HP: A certain percentage of HP is mapped to each leg, the torso and the head (for land critters). Exactly 20% is mapped to each leg, 10% to the head and 50% to the torso. For creatures with no legs, the leg hp is simply mapped to the torso and head instead (I can't figureout exactly how much goes to the torso and how much to the head, but it's definitely spread out). The exception seems to be the Walrus, which has 'invisivible' legs that can only be used in conjunction with the Walrus torso, but can be switched out for other legs. No HP is ever mapped to the tail, pincers/claws or wings. HP is magnified by size. A size 10 creature with all ant parts has 753 hp, as opposed to 30 hp for a size 1 ant (the combiner always uses the size of the larger creature). The amount by which HP gets multiplied seems to be constant for each size.
EDIT: For birds, front leg hp is mapped instead to the torso.
Defense: Defense works exactly like HP, except that it's unaffected by size. The only exceptions seem to be the snapping turtle, armadillo and lobster (partial credit for this goes to Once Known as X), where much more of the defense is mapped to the torso (note that using a snapping turtle torso does not garauntee 90 defense). Again, for creatures with no legs (excepting the Walrus), map leg HP to the torso and head. (Btw, the 'Defense' stat is a percentage of damage that will be removed, and horns/piercing reduce defense by 50%.)
EDIT: 25% of bird defense is mapped to the back legs, while 65% is mapped to the torso (as opposed to bird HP).
Speed: 40% of land speed is mapped to each leg, and the other 20% is mapped to the torso. For snakes, all land speed is mapped to the torso (the Walrus uses standard speed-mapping for its invisible legs). For swimmers, water speed seems to be unpredictable. For the Electric Eel, the vast majority of water speed is mapped to the torso, whereas for the Killer Whale (and most swimmers), the majority of water speed is mapped to the tail. Water speed is never affected by legs. 20% of air speed is mapped to the torso, and the other 80% is mapped to the wings. No air speed is ever mapped to non-fliers, no water speed is ever mapped to ground-only movers, etc. Speed is affected by size (higher size means lower speed), but not by very much; at the most dramatic size difference (size 1 to size 10), and using all the size 1 creature's parts, speed is reduced to 2/3 of the speed it would be at size 1.
Damage: Each attack is mapped to a certain body part, and melee damage is the sum of all melee attacks (artillery and ranged attacks do not mix like melee attacks do). Damage is greatly amplified by damage; in the most dramatic example, a dragonfly (size 1, attack 10) mixed with a sperm whale (size 10) and using all Dragonfly parts, gets you an attack of 42. Although there does seem to be some unpredictability; an ant, which has the same size and attack as a droganfly, mixed with a sperm whale only gets you an attack of 30.
EDIT: Relic actually made the way each creature changes with size different for balancing purposes.
EDIT: Sight radius: Sight radius is determined entirely by the creature's head, and is unaffected by size. It does appear to have an effect on coal cost. Flying units always have at least 25 sight radius (even if the creature's head has less), but do not get a bonus for heads with sight radii of 25 or greater.
That covers pretty much everything. If you have a correction ot request to make, or any comments to add, feel free to do so.
Edited to correct something
Edited again to correct something else
Edited yet again to correct a previous edit
Edited once more add some new stuff