I keep thinking about the "4-feet per chicken rule" for coops and it really seems a small area for a chicken. That is actually a 2'x2' area for each chicken to stand in and is often stated as the "minimum". Also, remember that feeders, waters, etc., will take up some of the coop area.
There again, the "10-feet per chicken rule" for runs again is the minimum normally stated. I'm thinking it gets up to ~140-feet per bird if you want to keep grass in the run...below that and the grass is going to suffer, down at 10' per bird the grass definitely disappears.