Sorry, Eagles has pretty got it spot on, some goldfish do really grow huge, some species grow in length bigger than my leg!
So the 0 on his keyboard is working perfectly fine, in fact he is talking of minimum tank sizes, so bigger tanks than the sizes he stated is better!
A common misconception is that most people think goldfish do not grow bigger than the tank they are in, and are easy to look after etc etc
Afraid not true, they are messy fish, meaning produces a LOT of ammonia and need a decent filter and regular large water changes and will grow stunted in small tanks, means body almost stays the same size but inner organs continue to grow, a painful way of living for fishes.
Also you cannot mix goldfish with tropicals either, goldfish do best in cooler waters while tropical fish are best in generally 24 to 30C.
Can't really have one with the other in either tank set up.