Side note: if you should decide you'd rather go with a smaller breed of goldfish, pet grade Demikin and Pearl Scale rarely exceed 5inches or so as adults. Short tailed Ryukin also have a high instance of producing a "dwarf" sized fish.
Example
I have a six yr old female Short tailed, bred her myself from large show quality parents, never been exposed to bad water in her growing yrs, and always fed high quality, but she and many of her siblings never got more than 2inches long. She's perfectly healthy, she's just tiny and adorable