Above two links will help you to configure and check the usage of the Flash Recovery Area (FRA) in Oracle. Now here is the method to resize the FRA in oracle database.
You can use below mentioned commands to see the already allocated space for the FRA.
1. Connect to SQL*Plus as sysdba
SQL> show parmeter DB_RECOVERY_FILE_DEST_SIZE
You can use following SQL to resize the FRA size
2. Connect to SQL*Plus as sysdba
SQL> ALTER SYSTEM SET DB_RECOVERY_FILE_DEST_SIZE = 250G SCOPE=BOTH SID='*';
Where 250G is the size of the flash recovery area size, SCOPE to BOTH will make the change to both memory and server parameter file, SID=’*’ has no effect in the single instance database where as in RAC database it causes the change to take effect across all instance.