Query DBA_EXTENTS for Extent number in the segment, Starting block number for the extent, Size of the extent in bytes, Size of the extent, in Oracle blocks : dba_extents « System Tables Data Dictionary « Oracle PL/SQL Tutorial

1. Oracle PL/SQL Tutorial
2. System Tables Data Dictionary
3. dba_extents

SQL> select
  6        Extent_ID,         /*Extent number in the segment*/
  7        Block_ID,          /*Starting block number for the extent*/
  8        Bytes,             /*Size of the extent, in bytes*/
  9        Blocks             /*Size of the extent, in Oracle blocks*/
 10   from DBA_EXTENTS
 11  where Segment_Name = 'segment_name'
 12  order by Extent_ID;
SQL>
SQL>