Oracle DBA, How To, Error, Cause and Action

To Check Schema Size 

"
SQL> SELECT owner AS "Schema Name", 
     SUM(bytes)/1024/1024 || ' MB' AS "Total in MByte"
  2  FROM dba_segments
  3  GROUP BY owner;

Schema Name                    Total in MByte
------------------------------ ------------------
WKSYS                          7.4375 MB
MDSYS                          46.375 MB
RMAN                           8.75 MB

Schema Name                    Total in MByte
------------------------------ ------------------
ORDSYS                         10.9375 MB
SYSMAN                         115.5 MB
TESTUSER                       .0625 MB
XDB                            208.3125 MB
SYS                            965.875 MB

Schema Name                    Total in MByte
------------------------------ ------------------
WMSYS                          7.125 MB

9 rows selected.