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...