Big, strong, and hard on crates
Rottweilers are heavy-boned, muscular, and physically capable of destroying a light-gauge wire crate in an afternoon. Adults run 80–135 lb, with males meaningfully larger than females. The 48" is the minimum for adults; large males need the 54". Buy heavy-duty construction rated for the weight — this is not a breed where you save money on the crate.
Rotties also chew, particularly as adolescents (roughly 8–18 months). A Rottie left in a crate with a fabric bed will shred and often swallow it. Bare pan with a rubber-backed kennel mat is safer until the chewing phase settles.