Not being rude but 28 gauge wouldn't be your best choice for ducks, but they have been killed with all gauges. Ammo is harder to find and you will have less pellets in the load compared to 12 or 20 gauge.
#4 shot might be the biggest you can find in 28 ga, but it will do. Non-toxic pellets (steel, tungsten) ranging in size from 4-1 is common place for ducks. For geese, I like BB but it all depends on your situation for both types of birds. Are you pass shooting, jump shooting, or over decoys? If over decoys I like to use an improved cylinder choke and 2's or 3's for ducks, but everyone has their own preference. I'm sure others will chime in after they get back from hunting today.