As QSECOFR or a profile with that authority, submit a job to run command RTVDSKINF.
This will build a file with all the information regarding object sizes.
I like to have this job run automatically on a weekly basis.
Once job is completed, use command (Interactively) PRTDSKINF.
You will be prompted for selection.
You can get summary and/or detail reports.
By size, by library, etc.