You want to learn which table is biggest in database or which schema is biggest in database.
You can learn Top Schema of database and Schema size with following script.
SELECT * FROM ( SELECT OWNER, SUM(BYTES)/1048576 MB from DBA_SEGMENTS GROUP BY OWNER ORDER BY MB DESC) WHERE ROWNUM < 20;
You can learn Top segment of database and Table size with following script.
SELECT * FROM ( SELECT SEGMENT_NAME,SEGMENT_TYPE, round(SUM(BYTES)/power(2,20)) SEGMENT_SIZE FROM DBA_SEGMENTS GROUP BY SEGMENT_NAME, SEGMENT_TYPE ORDER BY 3 desc) WHERE ROWNUM < 31;