The roost is usually around 48in but they will go higher given the chance. I had to block off my shelf and 8ft ceiling rafters. Just make sure the floor is padded well with bedding so they don't hurt their feet coming down. A general size for nest boxes is 12 inch square for standard breed chickens. The height depends on how you will be collecting the eggs. If your coop is raised and you have exterior access the boxes can be close to the floor. Mine are inside so I put them at 20 inches which seemed like the lowest I could go without having to bend to much to collect.
