Seeing your handle, "duck hunter", if you may be asking this question in reference to steel shot, the answer may be a little different. I know personally when waterfowl hunting with steel, I think it's really basic, other than you need to inject speed of the shot into the equation. Many waterfowlers feel that when using steel shot, speed of your shot is much more important than the load size. Since late season waterfowl are pretty thick with feathers and down, and since steel doesn't have the penetrating effect of lead, you can make up some of that disadvantage by increasing the speed of the pellets. Maybe giving a better penetration factor?
I believe it's a choice you have to make, either more pellets, or more speed. Some 1 1/4oz steel loads have a speed of over 1600fps, but step up to a 1 3/8 load, and your speed goes down to about 1340. I believe that is why 3 1/2" loads are becoming more popular. I normally never shot 3 1/2" shells, simply because they weren't really needed, but the 3 1/2" now offers you the best of both worlds. My shell for a normal late season duck or goose hunt is a 3 1/2" federal steel load, it's 1 1/2oz of shot, and is about 1450fps. So i'm getting a good amount of shot, but still have a good speed as well. Now you hunting situation should also tell you what to shoot, what I do, is try and find a shell that is the same speed, but maybe go back to a 3" shell or 2 3/4 shell when hunting early ducks, or close decoying ducks, if you know what i'm saying.
I'm no expert, and it probably doesn't make much difference, but common thinking to me that would be if you change the speed of your shell, I would think your swing and lead would change as well. That's why I try and stick with the same speed shell, you can compensate by using a smaller shell, or smaller shot, and not ruin the meat of early or smaller ducks, as the earlier post said. I feel sticking with a similar speed shell, it keeps my shooting more consistent, but that could really all be in my head, but that's what shooting is all about.