10 sq ft per bird is the rule of thumb you will typically find, so something in the area of 150 sq ft. as a minimum
*forgot to add that's a minimum...I'm a dolt...
Edited by tmarsh83 - 11/20/15 at 12:29pm
Make it as large as you have space for or as you possibly can. The stated minimum space requirements of 4 square feet in the coop and 10 in the run per bird are just that, bare minimums. You are much better off to go much larger. Your birds will be happier and you'll have far fewer stress/crowding related issues to deal with. Those problems are far easier to avoid in the first place then to solve once they get started.
If you go by the minimum standard of 10 square feet per bird in the run you are only looking at a run about 10 x 15 feet. I personally would not put 15 birds in that space, especially if they will be confined in the coop/run at all times. If they are allowed out to free range or have access to a larger fenced area like a pasture for much of the day then yes, you can get away with that size.