List all tables owned by a user sorted by size in Oracle

set lines 100 pages 999
col segment_name format a40
col mb format 999,999,999
select segment_name
, ceil(sum(bytes) / 1024 / 1024) "MB"
from dba_segments
where owner like '&user'
and segment_type = 'TABLE'
group by segment_name
order by ceil(sum(bytes) / 1024 / 1024) desc
/