The minimum I would suggest is 8" each if you have enough for averages to mean anything. I prefer more for a couple of reasons. When they get settled on the roosts, they really don't take up much room. But they spread their wings to get up to the roosts and they do a lot of moving around and repositioning once they are up there, so they need some extra room. What I mean by enough for averages to mean anything is if you have two chickens, 16" is not enough room for them to get up there.
The main reason I like extra is that some chickens can be pretty vicious on the roosts, especially when you are integrating new chickens. I like them to have enough room to get away from a mean hen. If some won't go up even if you have some extra room, this may be the reason.