Calculate the Size of an RMAN Backup Piece

DATAFILE_BLOCKS   The number of data blocks in the datafile
BLOCKS            The number of data blocks written to the backup
BLOCK_SIZE        The size of the data blocks in bytes

So BLOCKS * BLOCK_SIZE will give the number of bytes written for this 
particular datafile and hence the size of the backup piece may be calculated.

SQL> select bs_key, sum(blocks*block_size) 
       from rc_backup_datafile
       group by bs_key; 

If you do not use catalog (controlfile):

SQL>select bs_key, sum(blocks*block_size) 
       from v$backup_datafile
       group by bs_key;