I would create a LARGE rollback segment and prior to running the
large transaction ... set the transaction to use the large rbs instead
of extending the smaller rollback segment to death ...

sql> set transaction use rollback segment large_rollback_segment_name

Otherwise ... as Pando says ... Set the maxextents for the rollback
segments to unlimited (if you can't set the rollback segment as
shown above ... you'll get whatever one Oracle will give you)

HTH
Gregg