Length first for another grass parrot
A red-rumped parrot needs at least 36 inches long by 20 inches wide by 20 inches high for a pair, and an aviary or a flight cage stretching beyond that length serves it markedly better. The red-rump is an Australian grass parrot, and like its grass parrot relatives it earns its exercise in low, level flight and long sessions foraging on the ground, not in climbing a tall cage. A keeper choosing between a taller cage and a longer one at the same price should take the longer one without hesitation, because height above the flight line is the one dimension this bird barely uses.
Bar spacing of about half an inch to five-eighths of an inch contains a red-rump safely while letting it grip the wire, and a rectangular cage with a sliding, easy-scrub tray suits a bird whose floor is a feeding area. That floor needs active management: droppings, husks, and spilt water gather exactly where a red-rump eats, so choose a cage design you can clean thoroughly without dismantling the birds' whole home every time.
Set up the flight line and the foraging floor
Keep one unobstructed flight line from end to end, with a perch at each end set at the same height so the pair can fly level, land cleanly, and turn for the return trip. Red-rumped parrots are strong, eager fliers, and a bonded pair flying laps together is the clearest daily sign the cage is big enough. Scatter part of the seed ration on a clean floor tray or in a shallow foraging dish so the birds can work the ground as they would grassland, and put drinking water in a sheltered spot a ground-feeding bird cannot fill with husks in an hour.
Add a nest box only if you intend to breed, because a pair with a box becomes territorial about its end of the cage, and house red-rumps as a single pair rather than in a mixed indoor cage. Males in particular can be assertive with other grass parrots in confined space, a problem that shrinks in a true aviary with room to avoid each other and grows quickly in a cage that is merely adequate.
Indoor cage, outdoor aviary, and the weather question
Indoors, a 36-inch flight cage plus regular flying time in a bird-safe room can keep a pair well, provided the room routine is predictable and the cage sits somewhere calm with a covered, dark night. Outdoors, an aviary gives the flight this species is built for, but it must include a dry, draft-free shelter, because a ground-feeding bird in damp conditions picks up problems from a wet floor faster than a perch-living parrot does. In either setting, the dryness of the floor is a housing specification, not a housekeeping nicety.
Watch how the birds use the space you give them. A pair that flies mainly when startled, spends the day on one perch, or squabbles over the single good feeding spot is telling you the cage is functioning smaller than its measurements, whether because of furniture, placement, or a genuinely short flight line.
Buy once, measure properly, keep the record
Measure interior length with perches in place, check the bar spacing at several points, and favour a plain, long flight cage over a decorated upright one every time for this species. Write the final length, spacing, and floor routine into the pair's own record when the cage is set up, because with KinsLe's Transferrable Profiles a red-rumped parrot that is ever rehomed can carry that proven long-cage requirement to its next keeper, who can then house it correctly from the first day.