People buy fish at a variety of different sizes, depending on how old the fish are--some people will buy larger fish to go in a larger tank so the fish isn't out competed by tank mates. Others buy larger fish because they can be more hearty then younger fish. But don't buy a baby fish thinking that you can keep it at that size, as they will grow unless you stuff it in a 10 gallon tank with a dozen other fish, and then it will probably die.