it depends on where you live and what sort of goldfish it is. Also do you have a filter or do you need that too?
Generally adult goldfish should be housed in fish tanks that are at least 3ft long, preferably bigger. If there are a number of big goldfish then you want a 6ft tank or more. A couple of fancy goldfish would be fine in a 3ft tank.
Goldfish are pretty messy fish and need a good filter. You might find a decent filter will cost you as much as the tank, maybe more.