There is only one species of goldfish, carassius auratus. There are many different bred varieties such as the many orandas, ranchus, moors etc. All of these species can potentially reach 14" long and the record for a fancy goldfish is 17" !!
Taking this into account, You want a tank that is a MINIMUM of 24"x15"x15" for it to be totally happy. 5 Gallons is simply too small to hold a goldfish beyond 6 months realistically.
There is only one species of goldfish, carassius auratus. There are many different bred varieties such as the many orandas, ranchus, moors etc. All of these species can potentially reach 14" long and the record for a fancy goldfish is 17" !!
I think you are refering to commons or comets when you say they reach the 14" - 17" mark.
I dont believe fancies can reach 17" and most dont even reach 12". Those larger ones you see in shows are bred to be larger than the ones in pet stores and even then, they do not get as large as 17"