Edited by traildad - 7/23/12 at 8:39pm
If you figure 1 foot of roost space per bird, and keep the roost that is closest to the wall about 18" away, you should be fine. You can make all the roosts the same height, or stair-step them 12" across and 12" down. Roosts can be made from 2 x 4's with the wider edge up so that the chickens can tuck their feet under their bodies to keep them warm. The roosts should be higher than the nest boxes to discourage chickens from sleeping in the nest boxes and pooping in them. Many people use a poop tray under the roosts to make clean-up easier.
Hope this helps.