As an alternative perspective, let me say that my chickens have 15 sq ft per chicken indoors, plus their run... which, given that they spend a LOT of time indoors in winter, is about right, if you ask me. (Two of the pens have roofed runs, one of which is plastic-wrapped, and the other pen will get a roofed run built this spring and I hope to plastic-wrap *all* the runs for next winter. But still.)
I have tried keeping them with less space, but even though they have not self-destructed they really seem MUCH happier and more relaxed and more 'natural' with the extra space.
Mind, I just happened to have a large enough building to do this... still, even if I was building a coop, I honestly do not see myself giving them less than maybe 10 sq ft per chicken indoors minimum, under these conditions and for my tastes regarding chicken behavioral happiness. As with horses or dogs or anything else, sometimes it is better to keep fewer animals *well* than see if you can shoehorn more in.
JME,
Pat