as kgb suggested all too big

1. I was seeing lots of Redo log wait and Allocation retries related the Log_bufer. I read some book and it says by increasing the Log_buffer size I can avoid the Redo log wait and Allocation retries.

right but to get that part you have to understand how lgwr works which obviously you dont seem to be quite familiar, I suggest you to read Oracle Concepts guide from tahiti.oracle.com