Although it is not possible to resize most enemies, you can change other attributes for neat effects.
Transparent Enemies
Press T on an enemy to make it transparent like a ghost.
Flipped Enemies
Use F to flip an enemy upside-down. Assault Captains can flip themselves over after using their teleporter/cloaking device, but most of the other enemies work rather well.
Cardboard Enemies
Press R once on an enemy to make it wall-aligned (as flat as cardboard). It will still function normally! I don't recommend using floor-alignment mode on an enemy. Not only will the sprite look meshed, but it also causes them to float upwards indefinitely until they can no longer attack. These issues have been fixed in EDuke32.
Sunken Enemies
Press C on an enemy to submerge it halfway into the ground (as if it were wading in water). This creates the comical illusion of being chased by an enemy torso.
Coloured Enemies
Although shading won't affect an enemy, palette values will. You can use any palette value for variety, but some of them will have a special effect:
[1] = Enemy cannot be frozen.
[4] = Pitch black 'shadow' enemy.
[21] = Assault Trooper will become Assault Captain (with teleporter/cloaking device).
[>0] = Boss will become Mini-Boss affected by palette value (21 is the standard choice) (only the Battlelord is usefully compatible with this feature).
Note: The RECON (#1960) is not compatible with palettes. A palette value greater than 0 will instead designate it as a Multiplayer-only sprite.
Special Enemy Textures
Most of the enemies have a STAYPUT variant (for example, LIZTROOPSTAYPUT). These textures will cause the enemy to stay within their initial sector. This can be used to stop enemies from roaming all over the map, or to keep them in designated sniper positions, etc. There are other special enemy textures as well, such as LIZTROOPONTOILET, LIZTROOPRUNNING, LIZTROOPJETPACK, LIZMANJUMP, etc. Four special textures worth mentioning are EGG (#675), CANNON (#1810), RECON (#1960), NEWBEASTSTAYPUT (#4611), and NEWBEASTJUMP (#4690), because they're the only enemy sprites which are affected in-game by their sprite size in the map editor (this is quite hilarious because you can make them absolutely enormous or incredibly tiny). Check the "References - Special Textures" section for a complete list.