There is a bit more to know about the specific style of adjustable reamer.
Just to answer your question, if it’s a disc brake swivel axle the upper bushing size is 5/8” and the lower one 25/32”.
If it’s a drum brake swivel axle, the upper is 5/8” and the lower is 11/16”
Some retailers offer a fixed size reamer incorporating both diameters for one or the other style of swivel axle, however the fit is a bit loose in my opinion.
Dug