There's a mod called resize mod that enables you to set the chances of it being Small, Regular, or Giant.
It's percentage wise so let's say for zombie you do:
40% Small
50% Giant
That leaves 10% for it to be it's normal size, but most zombies will be Giant.
EDIT:
# ==================== resize mod - aka. Critter size variety ================
# Uncomment to enable mod:
#modResizeEnabled = yes
# Percentage chance for smaller than usual critter. [0..100]
optResizeCowSmall = 30
optResizeSpiderSmall = 50
optResizeSheepSmall = 30
optResizeSkellySmall = 10
optResizeZombieSmall = 30
optResizePigSmall = 50
# Percentage chance for bigger than usual critter. [0..100]
optResizeCowBig = 10
optResizeSpiderBig = 10
optResizeSheepBig = 10
optResizeSkellyBig = 20
optResizeZombieBig = 20
optResizePigBig = 10