I keep my three shubunkin in a 45 gallon tank. It is not ideal, but the chances of three $5 goldfish finding a home better than mine is very slim. They have four large internal filters going and their tank has been cycled for three or so years now. I do two 30% water changes a week and they get fed a varied diet of flake, frozen and live foods.
All three of my goldfish are around the 7-8 inch mark and seem to be getting bigger every day now.
I am starting to feel that their tank is getting way too cramped for them. It is not an issue of water quality with me, but just the space available for these fish to swim around in.
The shorter finned goldfish are so active. When mine are startled they can cover the whole 3ft span of their tank in a few seconds.
I think you are going to run into the same problem. If you are fastidious with your maintenance and overfilter, you should have minimal problems with stunting or water quality. However, your goldfish is eventually going to seem quite big for that volume of water.
I know the feeling of not wanting to rehome, especially goldfish. Once you relinquish control of your fish, you also relinquish control of their care. I have had my goldfish for three years and knowing their average lifespan, I feel the responsibility is the same as if I own a cat or a dog.
How do you know if a couple years down the track the person you rehomed your goldfish to doesn't move out and the new people tear the pond down? There's not exactly a great market for fish store goldfish.
So you kind of have to weigh up the two options. For me personally, the risk of rehoming and the 'unknown' is too great. So I have chosen to keep my goldfish. But for you, it could be a different choice.
I'm hoping once our renovations are done and I have my fish rack downstairs sometime in the next couple of months, I can look into getting a cheap 4ft/100 gallon tank.