Rollback segment question

Hey gurus,
 I have an index which is causing me problems and I have to rebuild. The index is on a very a large table.
 I am afraid I do not have a big enough Rollback segment to complete the operation. So I will have to create a new Rollback Segment and do a   set transaction use rollback segment

My problem is how do I estimate how big a rollback segment will the transaction require ? Any ideas , experiences will be appreciated.

                                 Tapan H Trivedi