If you want to estimate, and you are on 9i and above, try turning the DB_CACHE_ADVICE parameter to ON for a while. Then examine the data in the V$DB_CACHE_ADVICE view to determine whether you need to increase this parameter or not. Also, you would need to have an eye on the OS as well, since you don't want to increase this parameter too high so your OS page.
It also depends on the type of application. Since you mentioned partitioned tables, I believe this is a DW, on which you would probably won't a large buffer cache size, but instead fast disks to fetch the data asap.