Okay, I have 15 chicks that will one day be full grown chickens. What size coop will I need to build in order to house them if they are going to be free-range without a run? I've been trying to do the math myself, but its boggling. I thought about an 8x12 coop, but that might be too much for the number of birds I have. What's the smallest we can get by with. I have approximately 3 weeks to get this thing built.