The maximum Comet Goldfish size when fully grown is 12 inches long (assuming they have proper space to grow).
When you buy a Comet Goldfish, it’s usually a small juvenile that’s only a couple inches long. But that will change in a hurry!
As with any fish, their environment plays a big role in how big they can get. Poor water conditions can stunt their growth and shorten their lifespan. However, Comet Goldfish are a bit different.
It’s believed that the fish have a Growth Inhibiting Hormone that will affect their adult size. The hormone adjusts to the size of their environment. So, a larger habitat will result in bigger fish and vice versa.