A good "Rule of Thumb" for housing any large fish for life is to provide a tank with a length of, at least, twice the fish's adult length. The tank should also be, at least, half again wider than the fish's adult length.
Young aros are extremely flexible and can turn tight circles with ease. However, as they age (like all of us), they become less flexible and a wider tank will help facilitate a normal adult swimming pattern.
I realize that's asking for alot of tank to house a 48" fish. However, we're looking to properly house the fish. Not force them to conform to available housing.