I've settled on 2 3/4" shells loaded with 1 oz of #3 steel at around 1500fps. This will cleanly take ducks out to 40-45 yards, which is as far as most should be shooting anyway.
In the smaller gauges (16 & 20), I prefer bismuth or TM shot.

If you like shooting the 3 or 3.5" shells, more power to you.....but it's really not necessary.