This parameter defines the maximum size of save sets created by RMAN with an external backup library, that is, backup_dev_type = rman_util|rman_disk|rman_stage.
rman_maxsetsize = <size> [K|M|G]
Default value: 0 (no limit)
<size>: save set size in KB (default), MB, or GB
This parameter was previously called rman_setsize.
See also:
RMAN Backup with an External Backup Library
Oracle documentation