If you get the ducks in the decoys----a 20 gauge 3" magnum or 12 gauge 2-3/4" is sufficient.
Dave I would not rule out a 3" 20 gauge over and under. Forget the machismo many men with the little you know what syndrome will give you. A 20 Gauge O/U is cheaper to shoot and they handle like a dream. 3" 20 Gauge Bismuth is deadly on ducks at decoy range. When you take birds with something less than a 12 gauge ultimately you are respected.