Size category… have each size add to life rating.

[Delmuir]

Simple idea, each size above normal (or below) would add 1 to your life rating at level up. I'm not 100% how the formula works or if it's just 1 life but whatever… the point is, the bigger you're rated, the more life you get. It wold make holding on to items that increase size worthwhile.

[Zonk]

Only at level up? So it's not retroactive? If so...that'd be very awkard.

I'd rather make size give some sort of innate resistance to knockback and other similar effects, maybe based on size difference compared to the knockback-er.

[xnd]

Size is fun. Well you would be easier to hit, bigger target, defense penalty, but have longer reach. If it is just enlarging magic, that wouldn't add hp or weight, just relative size. Stealth penalty also: Easier to spot and more contact to make noise. Plus, big guy can't really swing his huge weapon well in small places, so any corridor with blocking tiles on both sides would give the big guy a penalty. Enemies can unload from afar upon his big form after anchoring him in various ways since they don't want to melee mr big guy. (my xorn got anchored by some rogues who kept teleporting me back to the anchor and firebombing me and poisoning me and phasing around me, oh that sucked). Plus you get a chance to hit your head every time you walk through a doorway. Pretty soon your new hit points are all used up.