Simplified 'combat at different levels' for size

[Anthony]

The rules for Combat at Different Levels in DF are greatly simplified from vanilla GURPS, which I appreciate, but then don't address different SMs. Here's a rule that is, I think, simple enough (and not 100% compatible with existing rules, but close enough):
Combat at Different Levels
In any situation where two melee combatants are at different levels, the higher one gets a benefit (due to gravity and positioning).

If your target is above your head, you get a -2 to hit, or -4 if it's at least 45° above your head (this replaces the active defense bonus). For a humanoid, vitals height is roughly (SM-1), torso height is roughly (SM-2); quadrupeds will generally have head, vitals, and torso all at (SM-2). This does generally favor attacking people's legs if you're below them.

If an attack comes from above your head, you get a -1 to defend, or -2 if it's from at least 45° above your head. Assume a melee attack is launched from vitals height; thus, on flat ground this usually only applies with a 2 or more point SM difference.

Regardless of angle, you cannot attack a target you cannot reach. Use common sense here, most cases that are large enough for this to matter are also multi-hex creatures and you'll also have to consider the creature's shape.