Ora-01555 error from a consistent export

(Oracle 8.1.7.4/SunOs 5.9)

A full export (consistent=y) from time to time gives me an Ora-01555. I have read two documents in Metalink(10630.1 and 1011108.6), both of which seem to be telling me to use a larger rollback segmnet. Now I have two questions:

1. Why doesn't the rollback segment grow when it runs out of space? The segment listed in the error message currently has only 12 extents when the maximum number was set to 32765. The db_block size is 16k which, based on Metalink doc 1011108.6, should limit the number of extents to about 1000. The rollback tablespace has about 1 gb of free space. All of this would seem to suggest that the rollback segment shown in the error message had room to grow.
2. Assuming that I need to assign a specific, large rollback segment to my export job, how do I do that?

Thanks,
Peter Schauss

--
http://www.freelists.org/webpage/oracle-l