The common rule is the bigger the better for goldfish, But I have seen so many hudge lionheads and Orandas that is kept in 5g tank as they cant handle the water current of bigger filtered tanks.. Very similar to Bettas
What will be your tank set-up? How many watts, substrate, ect...? Are you going for more of a natural/jungle look or is the plan going to be a zen garden look (which would look really cool with goldfish)?