I built my guinea coop as an 8'x12' with 8' walls. The ceiling joists are open so the guineas use them as their roosts.
Guinea coop is on left.
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I have 13 adult guineas in an 8'x12' coop and it is cramped quarters for them.
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In the heat of the summer the walk in door is propped open during the daytime to keep the coop from getting too hot. Both windows are fully open also.
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The bird door is 12"x12" but one that is 10"wide by 12" high would work just as well.
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My guineas spend most of their roosting time in the rafters. They do seem to like the lower roosts as steps to the rafters. The "nest box" in the lower right gets used for the first eggs of the year until they get tired of me stealing the eggs and they decide to hide their nests outside.
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They really do like their skylight.

My recommendation is to always build bigger than you think that you need. It will save lots of problems in the future.