Although it is not possible to resize enemies, you can change other attributes to create cool effects.
Transparent Enemies
Press T
on an enemy to make it transparent.
Flat Enemies
Press R
on an enemy to make it 2D (flat as cardboard). It will still attack normally! Note: I do not recommend floor alignment for enemies, they will slowly float higher and higher until they can no longer attack, and it looks stupid.
Coloured Enemies
Although shading an enemy won't affect it in the game, palette values will. Giving enemies different palettes can cause different effects. Here are a few known palette value changes:
Palette Effect 1 enemy cannot be frozen 4 pitch black "shadow" enemy 21 stronger enemy. Assault Trooper will become Assault Captain (with jetpack). Regular boss will become Mini-Boss.