Say a coop is 10 feet by 20 feet. How many standard chickens could fit in there comfortably? How many bantams? I don't want too many chickens, but Chicken Math is always there. Is this a nice sized coop?
How Many in a 10x20ft?
So looking at your worst weather - Winter, there may be days your chickens don't venture outside at all. So I would use something more like 10 sq. ft. per chicken, more like a max of 20. Your coop is still big enough for future division, like if you want to keep something small and docile as pets (like silkies separated from more aggressive large fowl), or a safe place for moms and babies before they are integrated into the flock.
Are you planning on just letting them free range without any predator protection or are you going to use something like electronet? I have to keep mine fenced or they would all have been coyote food long ago.
A covered area outside for the summer can be really helpful, a place to put food and water where it won't get rained on and a place to hide from hawks.
Good luck - there's a ton of good ideas on BYC that can help with. 10 x 20 is a great sized coop!