If the one your lookin into purchasing is still a juvie, you have a good 2 years before you'll need an adult size cage. I keep my hatchling-1y/o's in a 12 quart. Then bump up to a 28 quart until they are starting to fill it up. Then a 41 works wonders until you move up. I used 74 quarts before I finally switched to 4x2's.

So you don't need the adult cage right now, but keep it in mind. Bloods are fairly easy to spook. So a "smaller" cage works best for juvies/yearlifs. If they are in a smaller enclosure, they are more likely to tame out and eat with much more ease(from my experience). Not to cramp them, but a smaller enclosure makes them feel secure and starts your experience off on the right foot. For adults, 4x2' works but big females could use a little extra floor space(what it's ALL about for bloods). They don't move around a WHOLE lot, but they make AWESome captives/pets.
any more q's don't hesitate to ask or message.