Space questions are really hard to answer. It depends so much on your circumstances and how you manage them. Don't think of space requirements as only coop requirements. The space you are talking about is total space available to them, whether that is coop, coop and run, coop and free range, or something else entirely. It also matters when this space is available.
Commercial operations have proven you only need about 2 square feet per chicken if you trim their beaks so the can't eat each other or cage them in individual pens where they can't get at each other. Most of us treat our chickes better than this though. But if you are in a situation where you only use the coop as a place for them to sleep and maybe lay eggs and they have access to more space outside whenever they are awake, 2 square feet can work out really well.
Your management techniques play into this. If you want to sleep in occasionally instead of lettting them out at daybreak each and every day of the year, they need more space. If you build a predator proof run so you never lock them in the coop and the weather is such that they can get out and use that space year around, you don't have to worry about that.
I hate hard and fast numbers since there are so many variables. A general rule of thumb sometimes used on this forum is that if you provide 4 square feet per chicken in the coop and 10 square feet per chicken in the run, you will probably be OK with a really wide range of management techniques. I personally like to add more space, both in the coop and run. I find that with more space I don't have to work as hard and I have more flexibility in how I can manage problems.
I will mention that most building materials come in 4' and 8' dimensions. If you plan your building around those, you will probably have less material wasted. You can probably build a 8' x 8' for about the same cost of a 7' x 7', for example.
! I have 13 chickens, but 12 will be living in one coop! How big does it need to be?!?!?!!?!?!? I have 11 hens and 1 roo that will live in it! THANK YOU
