Personally, I refuse to put them in less than 4 sf per bird. Even my 90 yr old dad, who was a farm boy from way back, was just telling me they need 3 sf. each just the other day. Mine in the main coop have about 7 sf per bird right now, which is really about half capacity. That coop can hold around 40 birds, if you go by 4 sf/bird, but when I had 42 in there, it was a nightmare to keep clean. There are 20 in the 8x20 right now--trying to reduce numbers. I prefer a roomier area, especially when they must be kept inside due to really bad weather or when we have to go off for the entire day and aren't sure we'll be back before nightfall.
Much may depend on your climate, how secure the pen is so how often they really have to be locked in the building, etc.