I have mine in a 8ft wide x15ft long x 7ft tall pen, and they do fine. With pheasants though, the larger the better. I used to keep them in a 10x20, but the more common species really don't need one that size.
My goldens are in a 10X10... they do just fine.. its 9 feet tall, and has several branches uptop fro them to roost. They use the branches only in the evening and when they sleep. during the day,,, they use every part of there pen.