I'd say a 55 would be perfect. That gives the Goldies plenty of room to grow.
You could easily fit a bristlenose pleco in there, probably even two, and wouldn't have to rehome them since they stay pretty small, a 55 is actually a perfect size for them. Plus there are a ton of color varieties for you to choose from!
Filtration will need to be the biggest you can get, preferrably something that will cycle 4x the tank volume in an hour. Two filters is even better.
220gph+ is what you should look for, and adjustable flow rates would be even better so that you can turn it down if the goldfish get pushed around by it a bit too much.