pmfji, but why the poster can´t generate a parfile with the small tables, like :

set term off feedback off verify off pages 0 lines 500 trimspool on head off
spool pathandnameofparfile.par
select 'TABLES=' from dual
/
select decode(rownum, 1, '', ',') || linha
from (
select 'owner.' || table_name linha
from dba_tables
where owner in (listofowners)
and table_name not in
(list of big tables)
)
order by table_name
) a
/
spool off
exit

, export it, AND later generate another export script with only the (few, we think) big tables
using the syntax exp file=test tables=(listofbigtables) query='where rownum "<" 100000' ? After this the estimate is EASY, 100000 rows of big tables = x bytes in .dmp, SO total number of lines (knowed, one cant think) = y bytes....

Regards,

Chiappa