DB2 - How to estimate proper BUFFERPOOL size?
You can't "estimate" the size of a buffer pool, because the answer is, "it depends".

Try using the AUTOCONFIGURE command to get started, and enable STMM.  Together these will go a long way towards getting your database running more efficiently.

I would also recommend you spend some time reading about DB2 Performance Tuning.  There is a ton of information available on the web, starting from the entire Performance Tuning section of the manual.