i think you're thinking about this backwards... asking which goldfish stay the smallest is kind of like asking which kind of koi or arowana stay the smallest, or for saltwater, which sharks are smallest. it's the kind of fish where bigger is always, always better. even if you start out with a 20 gallon tank, you'll probably want to upgrade once your fish reaches adult size, if only to keep the water parameters stable. plus, big fish require big filtration and lots of food, which means lots of poo and big water changes.
Lexi_D, looking at your sig, you've got relatively tiny tanks there and a goldfish, with all its work, probably isn't the best match for you.