I really don't think the shot size matters a lot unless you are shooting at extreme range. If confining your shots to 50 yards or so, put the pattern on the turkey and he will be dead. I have killed them with 7, 8, 9, 9.5 and even 10.

I think the only real consideration for reasonable shots is that the larger shot will hold tighter patterns at greater distances than the smaller sizes. But I see that as primarily a disadvantage. Even the 10s have sufficient penetration to kill one at 50, so what's the advantage of a 7 at that distance? You just get a smaller kill pattern and a slightly bigger chance of a miss. You could probably figure out how to choke your gun to make them equal, but to me it's easier to just use 9s.

The only reason I ever loaded 7s was because NE wouldn't allow any smaller shot to be used. I see no other reason to use shot that big at turkeys. I could cite some things I've seen, but I will just say that 8s have enough penetration at even unreasonable distances. All 7 will do is just give you fewer pellets.