Welcome to the world of chicken, it is a wonderful place (to us on here at least) . I have read that 3 sq. foot per bird, but better is 4 sq. foot per bird of floor space in your coop. If it is real cold and snowy, your birds may stay inside most of the winter, it really depends on the breed and even then, some just prefer to roost most of the day. I have some die hard free-rangers and some who never leave the coop of the same breed, so there is no real way to know. I would always err on the side of TOO BIG, as well, you know, you can always add to the flock pretty easily, they often do this without much help by themselves. By coop of 12 X 18 houses 80 birds without much trouble, with most of them going out all day long. I have to add shavings about every week and fluff them up. It stays dry. I have 4 (12ft) roosts for them, so 48' linear foot of space, and my nest boxes are hanging on the wall. In the summer it is even dusty. I say, build as big as you can afford or have space for. I have a loft in my building, so it has better ventilation and stays cooler in the summer, but it is still really really hot in July & Aug. I have to run box fans in there. Show us some pics if you can. HenZ