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.
well the main problem is the rollback segment is too small...you can increase the size of them next time...but on the interim solution you can bring the rollback segment that is full offline and the next available rollback segment will take over and complete your export....but afterward try to increase your rollback segment.....