I would get a 3 1/2". It's the one time I still use a 3.5" shotgun, don't bother with geese anymore. If you are hardcore, only shoot one at 20 yards in the decoys, 3" is fine. I would definitely plan on a scope, so a receiver ready for a scope base is nice. I use a long eye relief, low-power scope. I don't do it for any other reason that some of these guns shoot so tight, that you just put a bead on the bird, you miss. With the scope, you can get it patterned and sighted. I went the red dot route at first, too unreliable for me. Batteries, broken electronics, etc.
My gun/shell/choke combo, shoots a pattern smaller than your fist at 20, so missing is real easy if you aren't prepared.