An American/Japanese shubunkin shares the same body style as the comet, and is classified as a torpedo shaped goldfish as opposed to the more egg shaped fancies. The single-tails can grow quite long, even the wakin (Japanese goldfish with torpedo shaped bodies and fantails) can achieve a good 10" or so.
IME, shubunkins and comets don't grow as quickly as commons but do catch up eventually. I'll try to take a picture of one of my sarassas this weekend for reference.
In the Spring you may wish to look into Rubbermaid stock tanks -- you could even set it up inside. They're very personable fish and quickly learn to eat from your hand and greet (beg) anytime you approach the tank/pond.
Shubunkin really are bred to be viewed from the top (actually, pretty much all goldfish and koi are with a few exceptions -- keeping them in glass boxes is a newer wrinkle in an ancient symbiosis) and, imo, are more appropriately pond fish tho plenty of people have kept them quite well in aquaria. I've seen some very pretty set-ups with simple stock tanks done very cheaply, if expense is an issue. (of course, there's always craigslist/freecycle/etc)