"the book" says 10 sf per chicken, of course more is better. I would suggest a 24 ft square run, with some thought as to how you might expand it later if needed.
The larger the pens get you'll find the less sqft per bird is needed. You'd not want more than 4 birds in a 10x4 run but as the run grows to 20x20 over 40 birds will easily fit and not be cramped at all. The much restated 10 sqft per bird is for small coops and runs. There needs to be a humane number for people with 4 or 6 back yard birds that may never leave the run and 10 sqft seems a good fit. As pens get larger the roaming space is exponentially bigger, the birds aren't getting any bigger. A chicken yard of 20x20 is 400 sqft, 40 chickens take up 60 sqft of space and 50 75 sqft. It's a difference of 340 to 325 square feet of roaming space. Not even noticed.
Thank you all for the insight. What about "Free Range" chickens? Can they still be some what contained or do they have to be totally free ranged?