SQL> show parameter db_recovery;
NAME TYPE VALUE
------------------------------------ ----------- -----------
db_recovery_file_dest string
db_recovery_file_dest_size big integer 0
SQL> alter system set db_recovery_file_dest_size="20G";
SQL> alter system set db_recovery_file_dest="f:\app\oracle\oradata\ora121\backups";
SQL> show parameter db_recovery;
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
db_recovery_file_dest string f:\app\oracle\oradata\ora121\backups
db_recovery_file_dest_size big integer 20G