Start bigger than the shop suggests
A crimson rosella needs at least 36 inches long by 24 inches wide by 36 inches high for a single bird, and that figure is a floor, not a target. Rosellas are built for distance: a crimson rosella in the wild commutes between feeding grounds on strong, direct flight, and a cage that only allows two wingbeats and a turn produces a frustrated, noisy, feather-worn pet. If your room cannot take a cage of that footprint plus daily flying time outside it, an aviary build or a smaller species is the honest answer.
Construction matters as much as dimensions. Choose a sturdy welded cage with bar spacing around three-quarters of an inch to one inch: wide enough for a rosella's grip and climbing, narrow enough to be safe for its head, and strong enough to stand up to a beak that tests welds and dismantles light fittings for entertainment. Powder-coated steel in good condition, a large main door for a bird that does not always want to come out past your hand, and a removable tray you can scrub are worth more than decorative shapes.
Furnish for flight, chewing, and bathing
Keep a clear flight lane along the cage's full length, with anchor perches at both ends and nothing bulky hung in the middle. Rosellas are enthusiastic chewers, so furnish with replaceable natural branches, bark, and shreddable material you expect to renew, and rotate items at the cage ends rather than crowding the airspace. A wide, shallow bath is not optional decoration for this species: crimson rosellas bathe with real enthusiasm, and a bath too small to splash in will simply be used as a second drinking bowl.
Place food and water where the main roost's droppings cannot reach them, and expect a rosella to carry food around and drop husks widely, so a seed skirt or an easy-clean floor beneath the cage saves daily friction. Site the cage where the bird can watch household life from a slight distance, because rosellas enjoy observing more than being in the middle of traffic.
Cage time versus flying time
Even a correctly sized cage is a base camp for a crimson rosella, not its whole world. Plan regular out-of-cage flight in a closed, fan-off, door-managed room, or move to an outdoor aviary with a sheltered, draft-free sleeping section if your climate and space allow. Pairs need still more room and are usually happier in aviary housing than in any indoor cage, especially outside the breeding season when rosellas can be assertive with their own kind as well as with other birds sharing the airspace.
Do not house a crimson rosella in a mixed indoor cage with smaller parakeets in the hope that its size will keep the peace. The same strong beak that needs heavy cage bars can injure a smaller bird in a single dispute over a perch, and separate housing is the standard this species expects.
Check the fit like a keeper, not a shopper
Before buying, measure your doorway and the cage's final spot, confirm the bar spacing with a tape rather than the box copy, and test that the tray, bath, and feeder doors all work with the cage assembled. Write the settled dimensions, bar spacing, and perch arrangement into the bird's record at setup, since with KinsLe's Transferrable Profiles a crimson rosella that is ever rehomed can pass that housing record to its new owner, who then knows the cage size this strong flier actually needs.