Long story short - bought a tiny 3ft x 2.5ft x2.5ft coop with attached nesting box when we brought our chicks home. We were planning to free range them in the back yard so minimal time in the coop. Realized it would be waaay too small for winter in MI and built a 4 x 8 x 5 ft tall coop with a little tunnel out the side attaching it to the original coop. Now our 5 chickens are almost full grown and they are cramped in the roost (in the small coop) and one always sleeps in the nesting box. So, I'm building a roost addition on the other side and of course midway through the project I am second guessing the size. It is 4 ft wide x 21 inches deep with a roof sloping from 24" to 21". Was planning to put a 2 x 4 roost bar down the middle, wood shavings on the floor for easy cleaning & soft landings, and cut a chicken size hole into the larger coop. 4 ft should be plenty wide, but will this be big enough at 21 in front to back for the birds to roost? We have a mixed flock, largest is an Austrolorp smallest an Amber Links. Would appreciate some advice.