ToME: the Tales of Maj'Eyal

What does increasing my size category do?

There are a couple of artifacts which care about size.

There's a Prodigy which might care.

Mostly, though, size does nothing.

Also Brawler Grappling (but it's still hated after all the reworks) and, I believe, Wyrmic Swallow.

Oh god grappling is not hated after the rework. It is definitely good now, and is probably the most common thing size will apply to for players.