That is a very good looking coop. I really like the slide out poo tray.
As coffeelady said, you dont want roosts over the nestboxes. I think most people have their roosts at 18 inches or more. I think thats what I read. Ours are probably 20- 24 inchesoff the floor with a ramp going up so they dont have to jump if they don't want to.( And good thing too. Ive found they don't want to jump)
We have 2 roost bars at different heights. They only use the higher one. The roosts are across the width of the coop ( 4 feet) and our 4 chickens use only half of it.
For our little chickens, we're building them a coop, and we'll have the roosts all on the same level. Right now they are in a temporary pen ( the shed) and they roost on their 10 inch high baby "playpen". There is 10 of them, and they huddle in a very small amount of space.
I could be very wrong, but I think the formula is 1 linear foot for each chicken. Someone please correct me if I am wrong