coop questions new to chickens
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Well, the slats of your goat pen, without adjustments would be pretty drafty. Most put ventilation in right under the roof on at least two sides, so their is air flow above where the chickens roost, but not on them. If you build a coop in the structure you wouldn't probably need to worry about drafts, you would just have to make sure it has ventilation.
You could do a coop for 6 birds that was like 4 x6 inside it, but that wouldn't account for getting new one's if chicken math hit. So basically anything that size or bigger would work, and all that covered barn would work as a nice run that they could probably use even in bad weather. I would just cover that one end with hardware wire if it were me.
If you haven't browsed this site for coop ideas, you should there is all kinds of them that can help give you ideas. I looked over a lot of them while building ours and it helped us a lot so that it was easy for us to clean/get eggs and keep the chickens happy.