My bantam coop is four feet high at its lowest point, going up to around four and a half feet high at the top of the slanted roof. The whole coop is itself on two foot tall legs so I don't have to stoop as I reach into the coop to clean it out, etc.
This gives me enough room to put a roost in there, yet not have the birds sitting in a draft while they're on the roost (the vents are up at the top of the front and back wall of the coop).  To me, that's going to be the deciding factor when you consider how tall the coop needs to be. Unless you have silkies, bantams can and do like to fly up to roost.
The tractor I have for these bantams is only 3 feet high, but doesn't have any roosts in it, since they are only in there during daylight hours.