Best to figure at least 4 sq ft per hen. so with 12 at minimum 48 sq ft of coop space. Depending on where you live and if you will free range or if they might be inside a lot you should consider more space in the coop. External nesting boxes (being able to access from the outside) are great, they don't take up room inside for the space. For roosting area depending on the breed (small-large) best to figure at least 10 inches per hen. They do like to move around on the roosting bar to get "their" best spot.
I'd probably go at least a 8' X 8' that would give you 64 sq. ft. inside, great size for 12 chicks and for when "chicken math" hits you can easily add more. Also having the nesting boxes outside is a great idea too. a good range of roosting bars is always nice too, especially if some of the one's you have are large breed hens, it may be a little hard for them to jump up too high.
There are a lot of pictures here of other coop designs to help with your decision.