All I had was a 16 when I lived in Nebraska, so that's what I used. It worked as well as my buddy's 12. No reason a 20 won't do fine.
Some 20's pattern more evenly with #6 than with anything larger, but the pattern board will answer that for you. I had better success with 6's myself, even with larger gauge guns.
A lot of people will tell you that a 12 is the only gauge there is for anything larger than quail. Sometimes the same guys consider the .30-06 the minimum caliber for centerfire rifles. However, one of the best turkeys I ever saw was shot by a beardless youth with a Mossberg 20.
Pheasant hunting is great sport. One of the things I dislike about SW MO is pheasant don't live here.
