Numerical size (1-3) helps determines melee range, target for missile collision and obstacle collision.
Descriptive size (small, medium or large) can determine chance of applying Knockback, and sprite size of a Grim Ward.
Cannot infer anything from sprite size.
Sub-tiles
Size
1
2
3
Size
1
5
13
Target
1
1
1
Collision
1
5
9
Obstacle
1
1
5
Unit size
Fangskin (size 3) is too large to follow the Werebear (size 2, despite his sprite size) up the stairs. If there were any size 3 monsters in the Arcane Sanctuary other than the The Summoner, they would have a similar problem.