I think you'll find a wide range of answers. Most helpful answer long run would be whatever patterns well out of your gun and fires at a speed you are familiar and well practiced with.
If I could consistently get decoying (i.e. feet down, backpedaling) I would shoot 3" #4's or even 2 3/4" #4s. I'm still working on the decoying birds piece, so I've started shooting 3" #2's @1550fps and seen an improvement in my shots per bird. I also like #2 shot because it's not unreasonable to take a shot at a goose with it should the opportunity arise - I'd want a goose to be extra close if I were to even think about firing with #4's.
Hope this helps.
