Mo I agree with teach1. As an example, when I built my coop (intended for 8 birds) I made sure my roost was 8'long. LOL, when I go out to do my headcount at night, I estimate the 6 birds I have are using about 2 feet of the roost.
I will pass on that most of the postings I have seen suggest that the roost (back side where the birds butts point) be at least 18" from the wall as it seems they will 'shoot' their poop and make the walls filthy if less than that. I still have a little problem on the side walls, as they all seem to squeeze to one side, but have not had a problem on the back wall. Don't set your roost too high in that coop as they will need head room when they jump up and there will be some wing flapping until they get settled for the night.