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I'd make them a minimum of 12" x 12". the maximum is up to you.
Suggestion: measure out 4 or 5 feet from the south wall, stack bales of straw/hay (if you happen to use them for other things around your place), or just build some panels out of chicken wire and 2x2's. Hang your nest boxes from the south wall, string a couple roosts higher than the nests, close off the top with chicken wire, and you're good to go.
I've seen this kind of setup used for a permanent coop for two-dozen chickens or more, depending on how many boxes and roosts you decide on. They just took the chicken wire, boxed in a section, added a screen-door for people access, cut a pop-door, and fenced in a small run on the other side. During winter, they'd stack bales of hay/straw for their other livestock against the chicken wire wall, helping to insulate it and cut drafts. Theirs was open at the top, and some of the chickens would go sleep with the horses and 2 milk cows sharing the barn. Eventually, they took down the chicken-wire wall, as well as the enclosed run, and the chickens were allowed the whole barn and yard, but would still lay eggs in their original nests.
HTH
Kathy
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