In my mind the problem not in size of rollback segment(s),
most probability, problem in not enougth RBS for new transaction during export.
Oracle use READ ONLY transactions in exp utility, and all RBS for uncommited and commited trasactions should be store all(or part) export time. If doesn't exists free RBS for new transaction, oracle generate error ORA-1555.

Problem in number :
#RBS(online) * TRANSACTION_PER_ROLLBACK_SEGMENT (init.ora)

and summary number transaction during export process.