I had a feeder get around 8 inches. It was around 8 years old when it died. I got it as a feeder when I was a bout 8 and put it in my 10 gallon and it started growing (i never changed water) and then i put it in a 55 gallon and gave it away. The lady I gave it too gave me the tank and the goldfish back 8 years later and the thing was huge!
This thread is hilarious to me. Here we are, all keeping of "monster" fish, which I'm guessing means "grows large, and maybe is 'monstrous' looking" and everyone says that a giant goldfish is ugly.....etc.
The Chinese and Japanese were the original monster fish keepers, don't forget about koi, which, to me, are more fancy goldfish. We keep goldies and love them. Personally, I would love to have a huge goldfish like that, they're not as smart as . . say.. . cichlids, that's for sure, but they all have endearing personalities
im working on getting a couple of ranchu to monster size. one of them is the size of my fist right now. i need a bigger tank if i'd like it to max out though.