I have a pen that is about 20 x 22. In that one coop are housed 43 birds. For that number, I'd say that's a good size run, but mine do freerange, too, most days for at least part of the day. When they have to stay in the pen, there's plenty of room for them.
20x22= 440 divided by 10 equals 44...
As far as the run goes, 10 sf is a good rule of thumb, so 12x8 pen would be enough for 24 birds or so.
My original coop was a simple design, 8x8, featured in the coops pages, with a shed roof, 6 ft high at the front to 4 ft high at the back. It's a classic, easy design to build. And build for...
The usual rule of thumb is 4 sf per standard size chicken for a minimum. At 5x5, that's 25 sf, so should hold six birds comfortably, maybe seven, but think of days when they cant get out due to really bad weather. The air quality will suffer with too many birds.
I think any coop should be off...