Узнать размер extent'a

set pages 999
col segment_name format a20
col "MBytes" format 999,999,999
select de.segment_name, sum(de.bytes) / 1024 / 1024 MBytes
  from dba_extents de
 where de.owner = 'HR'
   and de.segment_name = 'EMPLOYEES'
 group by de.segment_name
 order by 2 desc;

Результат:

SEGMENT_NAME             MBYTES
-------------------- ----------
EMPLOYEES                0,0625