This depends on the size of your tank. They can grow as large as ten inches in a pond. I purchased a feeder golfish 8 years ago and today in my ten gallon tank she is around 5 inches.
It depends on the age and breed. The longer breeds that we most associate with ponds (Koi, for instance) might grow to be longer than a foot. Fancy, round-bodied breeds can grow to 8 or 9 inches....
The maximum size of a fully grown common goldfish is almost two feet (60 cm) in length and weighing close to ten pounds (4.5kg). Most goldfish won't grow that large but can easily grow to around 12...
Relitely large. The ones you see at your petstores are usually young. It depends on the type, as comets grow 12 plus inches, some close to that, some around 6 inches. I hope this helps you.