SELECT e.OWNER, e.SEGMENT_NAME, e.TABLESPACE_NAME, sum(e.bytes) / 1048576 AS Megs
FROM dba_extents e
WHERE
  e.OWNER = 'MY_USER' AND
  e.TABLESPACE_NAME = 'MY_TABLESPACE'
GROUP BY e.OWNER, e.SEGMENT_NAME, e.TABLESPACE_NAME
ORDER BY e.TABLESPACE_NAME, e.SEGMENT_NAME;