ALTER ROLLBACK SEGMENT segname storage (maxextens 3000);

-- offline rollback segment
select segment_name, tablespace_name, status from DBA_ROLLBACK_SEGS WHERE STATUS = 'OFFLINE';

-- segment size
select segment_name, bytes/1024/1024 MB from dba_segments where segment_name='MY_ROLLBACK_SEGMENT';

-- take segment online
ALTER ROLLBACK SEGMENT MY_ROLLBACK_SEGMENT ONLINE;