Here I've created my own curve for how to roll Fort by applying a modifier to a roll of 3d6 (if anyone cares, the curve is based on the 4th power of the log of the mass), rather than the system Alexis uses that uses varying numbers of odd-shaped dice (which are rolled in Excel). I like the idea of needing only a few dice and a pocket calculator to figure out toughness of critters, and I also really like the idea of the bell curve being the same for all creatures.
Alexis's concept of incidental damage from big creatures is an idea of sheer genius. It perfectly encapsulates the terror and danger of fighting some huge beast. Read his post on it here. I really can't overstate how awesome this idea is. His story of watching a bull bucking and kicking, and how dangerous it would be to go near is a clear illustration of why any RPG with monsters need this system. I've tweaked his numbers a little, but would still go with a base 50% chance for anyone within melee range. Anyone intentionally keeping their distance would not risk the damage, but also wouldn't be able to make melee attacks.
I've also incorporated into this table the idea that small things are hard to hit, and would find it hard to hurt big things with some pretty honking penalties to hit and damage, and commensurate bonuses to Defense. If you think +16 Defense is too much, I'd like to see you try and whack a rat with a mace as it scurries about. Just picture that for a minute.
OK, time for a big table! I've tried to keep things fairly regular but still keep in with my idea of what makes sense.
|Mass||Avg. Fort||3d6 Multiplier||Incidental Damage||Attack/Defense/Damage|
This table would work just fine for D&D if you use the 3d6 Multiplier and instead apply it to whatever you use to calculate the HP of a normal human. You could either multiply the die roll, or multiply the number of dice. Obviously, if you go with multiplying the result, you'll get a very swingy total. If you multiply the number of dice, you'll get a very average result.