Creature sizes in Baldur's Gate 3 are split into a few categories: Tiny, Small, Medium, Large, Huge, Colossal, and theoretically even larger. Creature size in BG3 typically determines a number of stats and influences possibilities in combat, like one creature's ability to Shove or pick up another, but each size also comes with out-of-combat benefits (and drawbacks), too.
Baldur's Gate 3: All Creature Sizes (& How They Affect Skills)
Creature size in BG3 is a base attribute of almost every Baldur's Gate 3 creature, and determines stats like HP, AC, and the ability to Shove enemies.