Go as big as you can. I made my run 12x16ft for 15 birds and I still have pecking issues with 11 birds now. I don't free range. My coop is 8x8. All my birds are healthy, just a little feather pecked.
If you KNOW you're going to free range most of the time (taking into consideration times you may be out of town, or may need to keep them penned for other reasons) , then 150 sq. ft. would work fine. That way you won't feel too badly for them during those few contained times. But if there's a possibility that you won't be able to free range, then go much bigger, as large as you can. If you really think about it, 10 sq. ft is just a little bigger than a 3x3 space each, which really isn't much.