Metzer farms has a blog on managing waterfowl that Holderread contributes to. It's about as helpful as anything else I've found.
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X2
X2Most folks in books recommend 4 sq ft per bird but I like to advise going higher than that if you can, even if you free range. If you can't, the 4 sq ft is a bottom line to start from, but I wouldn't go any lower than that unless you have other options for them to exist in when weather is too bad to get out, like a covered run or lounging shed or some such. If you live where the weather doesn't get bad and you never have to confine the birds to the coop you can get by with less but I wouldn't overload it. Sixteen birds in an 8x8 gives you the 4 but since you free range you should be able to get by with it and the one extra...especially if you start a deep litter in your coop that can contain the fecal load and turn the coop environment into a healthier place to be, but remember if you go to deep litter to make sure you have really good, open air cooping...lots of windows and venting, both upper and lower.
I've got space for 20 using the 4 rule and I free range, but you see how quickly that space looks like a little box when they are confined due to deep snow...and I only have 9-10 birds in my coop most of the time now, so I'm giving them 8 and it still looks small. Crowding causes issues of squabbling, fighting, pecking, etc. and decreases the air quality of the coop, so I always advise to err on too much space rather than just enough or too little.
Those recommended numbers are minimum guidelines. More is always better.
I tell people if they are buying a manufactured coop, ignore the number of birds the manufacturer says it will house, their numbers are nonsense.
Measure the interior floor space and then use the 4 sq. ft. thing as your guide. Most of them will hold about a quarter of what they say they will.