I just finished a new coop I built for my girls. When I first got my flock I inherited 8 hens from my neighbors when the moved along with the hens they gave me the run and coop they built. They had 8 hens in a 4x4 coop with no ventilation. I made as many modifications as I could get get...
It has been my experience that most of your prefab coop are rated for bantam size chickens and not a full size chicken. I agree with the others, keep it to at least 4 sq ft per bird.