--
-- add the where clause tablespace_name=yourtablespacename
-- segment_name should be your TABLE_NAME/index_name 
mag@mutation_mutation > get size
  1  Prompt size of tables
  2  select sum(bytes/(1024 * 1024)) "size_in_megs" from dba_segments where segment_type='TABLE' and owner='MAG';
  3  prompt size of indexes
  4* select sum(bytes/(1024 * 1024)) "size_in_megs" from dba_segments where segment_type='INDEX' and owner='MAG';
mag@mutation_mutation > @size
size of tables

size_in_megs
------------
       .3125

size of indexes

size_in_megs
------------
       .0625