I researched a lot for sizing our coop, there were so many different ratios and opinions. In the end, we went with 4 square feet per bird inside the coop and 10 square feet per bird outside the coop (including the area that can be well-used under the coop if there is adequate height. And yes, plan the dimension carefully to maximize the available lumber sizes. After observing our 4 hens for 3+ months, I would say this particular "rule of thumb" works great. Any smaller space would have been too crowded for the chicken activities. The only thing I would change if we were to do our coop over is the height. We should have made it at least 6 ft high in the lowest part. Bending our heads to clean the run or to feed the chickens was a pain.