i have a buddy that used a 20 ga. for ducks and geese for the first 2 years that he hunted. his gun was also chambered to fire only 2 3/4 shells and he could drop them like rain. the shot you use plays part in it but the biggest factor is knowing your range and getting the pellets into the kill zone. that being said, personally i would use either 2's or 4's because you shouldnt be shooting that far anyways. and if you have interchangeable chokes then a modified would probably work best however you should pattern your gun with some different combinations of shot size and choke to find your ideal set up.