Ok...
Its the common goldfish and comets that are known to get to 15 inches, not black moors and orandas. Id stick with getting the large tank though, as orandas do get quiet large still and therefore producing alot of waste.
Would have been more helpful by the way by saying what type of goldfish you had at the start, as when you say goldfish everyone assumes that your talking about the lil fish that are known to go in bowls but turn out to grow massive! Two different types of fish are goldfish and the fancies.