This page outlines things to consider when choosing fish to match your tank or a tank to match your desired amount of fish.
Goldfish are extremely dirty fish. They produce a lot of waste and will often eat whatever fits in their mouth and goes down. If you don't want to have to clean your tank and perform partial water changes very very frequently, make sure you get a large tank. Goldfish can grow very big and the tank you start out with can become too small more quickly than you may realize. For every inch of goldfish you have, you should have 3 gallons of water. Not only because they're dirtier than other fish but because they can grow to be surprisingly large. Comet goldfish can grow to be over a foot long if given the proper conditions. There's a tendency to believe that goldfish will only grow to the size of their tank, but many of their organs do not stop growing and this will often kill the fish. If taken care of perfectly, a goldfish can live for upwards of 20 years. If not allowed to grow, serious problems can occur.