2-3/4" #4's dropped a hell of a lot of ducks for me on the bays and in the marshlands on the coast.
I rarely saw #3's in the store, and #2's were a little too heavy, so #4's became the go to most of the time.
I always had a box or two of 3" in the bag to swap over if the birds were flying high or weren't decoying all that well, but probably better than 90% of the time the 2-3/4" shells were far more than adequate.
Goose loads were a different story.
I suppose if you are teal hunting and have good shots, #6 of bismuth, tungsten or hevi-shot would work pretty well, but i wouldn't personally shoot anything that light at larger ducks (mallards, pintail, redhead, etc.). I'm sure plenty of people do with success, but not my cup of tea.
A lot of your load choice has to do with where you are hunting, what he dominant bird selection is, and what their flight and decoy patterns are. We had one blind that just about required 3" shells - for whatever reason the birds didn't decoy as close to this blind as we wanted and the longer shells made a huge difference. When we built our other blind, conditions generally brought the birds in a lot closer, and the 3" shells did more damage than anything, so the 2-3/4" became the go-to choice for that location.
If you are hunting wood ducks and teal in cover, 3" shells are usually overkill. A buddy of mine from A&M used to hunt the Scatters up in Arkansas around Pine Bluff (lots and lots of flooded timber) and never used a 3" shell, because they simply didn't need them where he hunted at.
At one of my old friend's place near Caldwell you needed both, depending on where you chose to hunt - flooded timber was 2-3/4" with limited shooting lanes, the pond was a 3" shell type of blind to reach to the other side of the water, and even then it was a little iffy.
But still, it always depends on specifics.