Bear in mind that goldfish get HUGE.

They can get just as big as Koi. A friend of ours started out with about 10 little tiny goldfish (like an inch long) in a 25 gallon. In less than 5 years the tank was overcrowded and on of the fish was about 6 inches long. She gave the fish to a friend with a pond where they are happily living and now even bigger.