The only real way is to watch your online redo logs in action and use that information to size them appropriately. But I can make some recommendations.
First, start with five redo log groups. A good size I use is 100 MB in size for each group. Then, watch your database in action. You should not see any "checkpoint not complete" messages in your Alert Log. If you do, consider increasing the size of your online redo log groups. I recommend switching the online redo logs no quicker than once every five minutes. If the online redo log groups switch more often than that, you will need to increase the size of the groups. I also like my groups to switch more often than once an hour for non-quiet times. So if you are not switching often enough, consider reducing the size of your online redo logs.
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