This parameter defines the maximum size in kilobytes of the backup pieces created by RMAN with an external backup library, that is, backup_dev_type = rman_util|rman_disk|rman_stage.
Syntax: rman_maxpiecesize = <number>
Default: 0 (no limit)
You can also use this parameter to limit the size of a save set created by an RMAN incremental backup to local or remote disk.
This parameter was previously called rman_kbytes.
See also:
RMAN Backup with an External Backup Library
Oracle documentation