It depends on how far you want to shoot them
2's, and 3's(any length) work well for pass shooting ducks.
4's, 5's and 6's work well for shooting over decoys if you keep your shot in close. A duck passing your decoys 50 yards out isn't a decoying duck. The 6's work nice on early season teal.
Save the 7's for rails and snipe.
