I'm really getting stumped on the disparate sizes of potential mutant animals, with characters potentially ranging in size from smaller than mice to larger than elephants, how do we address the issue of characters hitting varying sized characters (and the respective damage their might do)...and when I do this, how do I keep it from getting too crunchy when the rest of the game is predominately narrative in spirit.
At this stage I'm thinking of allocating a limited "size class" mechanism.
Each character has a size class from 0 to 14 (typically), where 7 is a regular human (50-100kg, 1.5m-2m) and every increment basically halves or doubles the weight (and modifies height by about 30%). The whole system is logarithmic.
In this way we can compare the difference between character sizes, and the same effects apply. A size class 3 character facing a size class 6 character (3 levels of difference), works much the same as a size class 6 character facing a size class 9 character, or a size class 10 character facing a size class 13 character (both of these are also 3 levels of difference).
If we use the FUBAR system of combat, three levels of damage neutralise a character (completely removing an unnamed character, or temporarily paralysing a named character), and six levels eliminate a named character from the story completely.
Perhaps a successful hit could score an automatic level of additional damage for every difference in size (or subtract levels of damage if targeting larger opponents).
Perhaps we simply offer the chance of an extra level of damage after a successful hit if a larger attacker rolls a d6 and gets a score less than the difference in "size class". Or they lose a level of damage if a smaller attacker rolls less than the difference.
I'm not sure, it feels a bit messy.