SELECT a.table_name,
a.tablespace_name,
ROUND(SUM (b.bytes) / 1024 / 1024, 3) gigabytes
FROM user_tables a,
user_extents b
WHERE a.table_name = b.segment_name
AND a.tablespace_name = b.tablespace_name
GROUP BY a.tablespace_name, a.table_name
ORDER BY gigabytes DESC, table_name ASC;

This will show all the tables sizes in a schema. Modify it to show only the table you want.