A 20ga is more than enough for both ducks and geese, PROVIDING, they are close enough, decoying/decoyed, and you are shooting the right shells.
I shoot a 20ga and Steel Remington 3", 2's or 3's, for ducks. I shoot my 20ga in Wisc. for geese all the time. I shoot Hevi-Shot 3", 4's or 2's for geese. All birds are shot with a Imp. Mod choke and all birds are decoying/decoyed. All shots are within 30yds. Birds are DRT.