“you can get “ACTUAL UNDO SIZE” from :
SELECT SUM(a.bytes) “UNDO_SIZE”
FROM v$datafile a,
v$tablespace b,
dba_tablespaces c
WHERE c.contents = ‘UNDO’
AND c.status = ‘ONLINE’
AND b.name = c.tablespace_name
AND a.ts# = b.ts#
“
By Singh (http://forums.oracle.com/forums/thread.jspa?threadID=519539&tstart=0)
