First find the block size:

SELECT VALUE "BLOCK_SIZE" FROM v$parameter WHERE name=’db_block_size’ ;

Now check the usage of the tablespace:

SELECT  s.username "USER",s.sid,s.osuser,  u.TABLESPACE "TS" ,  SUM(u.blocks) * &BLOCK_SIZE./1024/1024 MB,  x.sql_text FROM v$session s,v$sort_usage u,v$sqltext x WHERE s.saddr=u.session_addr  AND s.sql_address=x.address GROUP BY  s.sid, s.username, osuser,  TABLESPACE, sql_text, address, piece ORDER BY sid, piece ASC; […]