Find Oracle DB table size

To find the size of all the tables in a DB:

select owner, segment_name, sum(bytes)/1024/1024/1024 from dba_segments group by owner, segment_name order by sum(bytes)/1024/1024/1024 desc;

To find the size of a specific table in a DB:

select owner, segment_name, sum(bytes)/1024/1024/1024 from dba_segments where segment_name='&segment_name';