Here is a query, you can run it in SQL Developer (or SQL*Plus):
SELECT DS.TABLESPACE_NAME, SEGMENT_NAME, ROUND(SUM(DS.BYTES) / (1024 * 1024)) AS MB
FROM DBA_SEGMENTS DS
WHERE SEGMENT_NAME IN (SELECT TABLE_NAME FROM DBA_TABLES)
GROUP BY DS.TABLESPACE_NAME,
SEGMENT_NAME;
If you don't have DBA rights then you can use user_segments table:
select bytes/1024/1024 MB from user_segments where segment_name='Table_name'
Source: Stackoverflow.com