I always go by what I have read in good chicken books, 4 sq ft for large, 3 for medium and a bit less for small. Fudge a bit if you freerange every day and if you have a very large pen. It seems to work for me.We downloaded the design specs. For this model back in March and it is how we had built our coop- except we doubled everything thinking it would fit six comfortably. This website says that the original design size can house six chickens, it just seems way small for six- even ours where the size is double seems tight sometimes for our big breeds. What do you think? Thank you.