Won't tell you what you should use, but for my gun, and where I hunt, I use:
2 3/4" or 3" shells in an old Maverick 88 with a Modifed Choke.
Lead Shot - #4 for Ducks, #2 for Geese
Steel Shot - #2 for Ducks, BB for Geese
I prefer 3" for Geese, and 2 3/4 for Duck - mostly due to distances being shot ... we "puddle jump" the river in a canoe for ducks, so most shots are 20 yards or less, and Geese tend to be in the bigger water or fields, so the ranges are a bit further (up to 50 yards) and need that little "extra" to be efficient.
Somethig you may want to think about if you will be hunting from a canoe instead of a fixed blind ... grab the heaviest load you expect to use, put your canoe in soem water, and take a half dozen "practice" shots to get the feel of shooting at various angles and positions ... While shotgun recoil is fairly negligable most of the time, there is an added degree of difficulty when you are twisting your body around to track a bird in the air while sittin in a narrow boat. Between being in an uncomfortable shooting position and the recoil ... well ... it's an experience best learned when your prospective dinner isn't laughing at you as it flies away
