My brother in law made his about 12"x12" and it works just fine for him. Mine is much larger, about 2'x3' because I've got 40 of them all crowding to get out at the same time. So, somewhere between there should be fine.
Mine is just under 16 inches. I built the coop myself and the wall studs are all 16 inches on center. So instead of having to frame in any extra boards I just cut out an area between to of the wall studs.