If you are not using external archive media, then the size of the recovery journal is critical for data protection. Rollbacks cannot extend beyond the logs that exist on the recovery server. You must estimate average throughput and calculate journal area based on the length of the desired average restore window.
The size of the recovery journal is proportional to write throughput and the required internal rollback window.
The journal on the recovery server should be at least 256MB. Note that this is twice the space recommended for the minimum on the production server, because the recovery server contains both redo logs and undo logs.
Refer to Planning your Environment for information and formulas to help you size the journals for your system. Refer to Using the Sizing Tool to Calculate LFC Size to automated the process of sizing the journals for your system.