These are the offcial base sizes for Games Workshop’s Mordheim:

Mercenaries
All models: (20 x 20 mm)

Witch Hunters
All models, including Warhounds: (20 x 20 mm)

Sisters of Sigmar
All models: (20 x 20 mm)

Cult of Possessed
Possessed: (40 x 40 mm)
Beastmen: (25 x 25 mm)
Magister, Brethern, Darksoul, Mutant: (20 x 20 mm)

Undead
Dire Wolves: (20 x 50 mm)
Vampire, Necromancer, Dregs, Ghouls, Zombies: (20 x 20 mm)

Skaven
Rat Ogre: (40 x 40 mm)
Other models: (20 x 20 mm)

Hired Swords
Ogre Bodyguard: (40 x 40 mm)
Freelancer Mounted: (20 x 50 mm)
Pit Fighter: (25 x 25 mm)
Halfling, Warlock, Elf, Dwarf: (20 x 20 mm)

Dramatis Personae
Aenur, Bertha, Johann, Veskit: (20 x 20 mm)

Mounts
Horse / Warhorse: (20 x 50 mm)