Let me start by stating that Oracle recommend Redo log size on Exadata should be >= 4GB and less than 32GB. You should not have redo log size greater than 32GB unless instructed by Oracle Support. Oracle recommend minimum 4GB redo log size because Exadata machine can support extreme IO rate. With latest Exadata X-6 you can perform up to 5.6M 8K database read I/O operations, or 5.2M 8K flash write I/O operations per second per full rack.
If you application is generating a lot of redo logs, 4GB might not enough and consider increasing it. Undersize redo logs can lead to unnecessary log switches and check point activities causing performance bottlenecks. You can start with Oracle recommended 4GB size and observe log switch activities before you decided to increase to decrease redo log size.
Ideally online redo log files should be the same size and configured to switch approximately once an hour during normal activity. They should switch no more frequently than every 20 minutes during peak activity. There should be a minimum of four online log groups to prevent LGWR from waiting for a group to be available following a log switch. A group may be unavailable because a checkpoint has not yet completed or the group has not yet been archived.