Shotguns are typically thought of as being used on flying critters and so are traditionally loaded with "birdshot." Birdshot has size designations from No. BBB to No. 12. Sizes from BBB to No. 9 are considered "birdshot" while No. 12 is sometimes called rat-shot or snake-shot and is typically found in rimfire loads and some centerfire pistol offerings. Probably the most popular sizes range from No. 6 to No. 9 - the higher the number, the smaller the shot but the larger the number of individual shot in each shell.
The 410 has factory offerings loading shells with No. 4 to No. 9 shot. It is to be remembered that the small size of the 410 shell lessens the number of pellets per shell and requires the shooter to be really careful to aim correctly to put the smaller pattern on target.