Ballistically 3 inch #3's are the best "go to" shot for ducks as they have enough weight to carry the kinetic energy needed to kill at approx. 30 yards plus enough pellet count to ensure a few hit the bird. Research showed it takes an average of 3 pellets to do this but in reality I've had to do a follow up shot on a lot of my birds... go figure ? In the old days of Lead shot we used #7 1/2's to swat the cripples on the water so I could see using #7 Steel shot for that purpose and of course it would make a fine load for Doves or Woodcock. For early season birds #4's may work as the birds haven't got thicker feathers/down yet but if you buy your shells by the case why bother ? Come November the birds are bigger and better armoured so bumping up the shot size ( #1's or #2's ?) for ducks is a good idea but as for geese I just use BB's all season. I do know guys that use #2's from the beginning and step up to BB's later on but they like to shoot further out then what I'm comfortable with and their logic is "if I hit them they're dead". So finding the right size and load and practicing/hunting you will find that "one shot kill zone" that is right for you.