Tamil has given a work around.

To avoid ora1555:

1) Increase the size of rollback segment. (which you have already done)

2) Process a range of data rather the whole table. (need to do in your circumstances)

3) Add a big rollback segment and allot your transaction to this rbs.

4) There is also a possibility of rbs getting shrunk during the life of the query by setting optimal. (ruled out in your case)

5) Avoid frequent commits.

6) google out for other causes.