The structure suited is of the wargame "Siege of Jerusalem" from Avalon Hill
A legio is composed for 6 cavalry units, 30 legio, 3 archers, 9 vellitae, 6 foederatti, 3 assault towers, 1 ram, 4 catapults-scorpio, 1 catapult-onager, 1 catapult-ballista and headquarters. Contain a 4 x 3' terrain-city map, 4 legio, and 6 factions of judean armys and militias.
I have calculated a army with 180 legio, 54 vellitae, 36 foederatti, 18 archers, 36 cavalry units, 6-8 scorpio, a onager, optional carroballista, 4 towers and divers headquarters.
Reducting centuria units (x 1/2 to x 1/4) make possible to complet a aparent cohors