I just went out and measured mine so I could give you the accurate height that I used.
The least amount of room I have in one of my tractors is 18" from roost to ceiling. But that is for my short Araucanas.
The Barnevelders I made 2' and they often just stay on the floor. I have wondered if they wanted more head room. They are big chickens.
I have my Jersey Giants in a large coop at night since I couldn't make a tractor big enough for those whoppers. Their roost now is 4' from the ceiling.
My highest roost gives them about two feet clearance above them but I've seen them squished happily into the space where the roof meets the walls with no space above at all. Who knows what lurks in the hearts of chickens?
Jan la Banan