Staring at IBM i 7.2, it is now possible to save libraries by size when using the SORT parameter in  BRMS control groups or SAVLIBBRM command.

Using the Sort By (SORT) parameter specifies the order in which the libraries specified for the Library (LIB) parameter are saved.  If *NONSYS or *ALLUSR is specified for the LIB parameter, libraries QSYS2, QGPL, QUSRSYS, and QSYS2xxxxx (where xxxxx is an independent ASP number) are saved first, if they are located on the ASPs specified by the ASPDEV parameter.   

*NAME                                                    
    Libraries are saved in the order the names are specified for the LIB parameter.  For each generic or special value, libraries on independent ASPs are saved  before libraries on the system and basic user ASPs. Libraries on the same ASP are saved in alphabetical order by the library name.
                               
*SIZE                          
    Libraries are saved in the order of the total library size, from largest to smallest.  This may improve the  performance of the save operation when using multiple tape drives.     

Restrictions