I think vertical space is more important for ventilation and ease of cleaning. I built a couple of 4ft. X 8ft. coops. They have 4ft. walls. I added a steep pitched roof so that I can stand up in them. I am 6ft. tall. The roof is 6ft. 6in. tall at the bottom of the peak. I added eave and gable vents which are above where the chickens roost. These coops are much easier to clean. Than the coop that I have to clean while leaning over in the doorway.