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.