When I used to duck hunt, I used #2's.

I hunted a swamp that my dad used to hunt since he was a kid for woodies that is supposedly owned by someone else, but I hunted off a set of abandoned railroad tracks which is public. The "owner" confronted my family after a duck hunt saying we couldn't hunt it and never denied the fact that we could hunt it from the tracks. We just stopped hunting there because we just don't want to have to deal with her. That was the only place worth hunting for ducks in our area.

My dad brags about how he used to be able to hunt with lead shot #6's for ducks. Oh well, so much for the good old days. Since we have stopped duck hunting, they have come out with Hevi-Shot. If it performs better than lead like they claim and you feel like paying a little extra, you could probably use that in #6's.

Although the swamp I hunted was fairly open in some spots, I never wacked a duck beyond 20 yds. Then again I was using Improved Cylinder.