What is the minimum tank size for a 12-14 inch common goldfish?
One gallon of water per inch of fish is the rule of thumb for small tropical fish. A 14 inch common goldfish is not common.I think he needs a small pond or a 50 gallon tank. He wouldn't have room to move in a 15 gallon tank.
They should be able to swim at least 3 body lengths and turn around easily. That sounds big for a common goldfish. Is it a Koi? Those will need more room, and will still have some growing to do.