For ducks 2 3/4 shells. If you can’t knock them down with those you are not going to going to get them with 3 inch shells either. No reason to buy 3 inch shells. The difference between 2 3/4 in and 3 inch shells in #2’s is typically 9 pellets, depending on the manufacturer and load weight. My point is, inside 35 yards, 9 pellets in your pattern aren’t going to make a difference.
Sizes in steel #2, 3, or 4’s. Buy what is on sale. Buy what you can get.
Early season I shoot 4’s due to having a lot of teal around. Shots tend to be close on bigger ducks. Wind tends to be light.
Late in the season I shoot 2’s. Not many teal around. Usually hunt bigger water. Shots are longer. Winds tend to be stronger.
Can’t decide, buy 3’s as they work great.
If geese are a good possibility while in the duck blind I usually keep a box of 3 inch BB’s or BBB’s handy. If we see geese coming from further out, I will rapidly reload with the goose loads. If not I shoot 2’s. When your target may range from geese to teal, it becomes a compromise. You need to be prepared to shoot what is in your gun, without the luxury of having time to change your loads out.