This script will spool out the index create statement with the true size 
of the index.

variable block_size number
begin
select to_number(value) into :block_size from v$parameter where 
    name = 'db_block_size';
end;
/

drop table t_size;

create table t_size
(owner      varchar2(30)
,index_name varchar2(30)
,used_blocks number(9)
)
tablespace users
storage (initial 64K next 64K pctincrease 0)
pctfree 0
pctused 80;

declare
v_cursorid integer;
v_dummy    integer;
v_indx     varchar2(30);
v_next_extent number;
v_owner    varchar2(30);

cursor c1 is 
select owner,index_name,next_extent 
from dba_indexes 
where owner not in ('SYS','SYSTEM');

-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 
-- find true size of each index - store in t_size
-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 

begin
v_cursorid := dbms_sql.open_cursor;

open c1;
loop
   fetch c1 into v_owner,v_indx,v_next_extent;
   exit when c1%notfound;

   dbms_sql.parse(v_cursorid,'analyze index '||v_owner||'.'||v_indx||
   ' validate structure',dbms_sql.v7);
   v_dummy := dbms_sql.execute(v_cursorid);

   insert into t_size 
   select v_owner,name,lf_blks+br_blks
   from index_stats
   where ((blocks-lf_blks-br_blks)*:block_size)> v_next_extent;
end loop;
dbms_sql.close_cursor(v_cursorid);
close c1;
commit;
end;
/

-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
-- spool out the create scripts
-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 

spool c:\clients\pg\alter_indexes.sql

select 'alter index '||a.owner||'.'||a.index_name||
       ' rebuild tablespace '||
        tablespace_name||
        chr(10)||
       ' storage(initial '||(a.used_blocks*:block_size)||' next '||
       b.next_extent|| ' maxextents '||b.max_extents||')'||
       ' unrecoverable;' 
from t_size a
    ,dba_indexes b
where a.owner = b.owner
and a.index_name = b.index_name;

spool off

@c:\clients\pg\alter_indexes.sql

drop table t_size;