Nope. You don't define the size of the buffers - they always match the blocksize, instead you define the number of buffers which should be allocated.
But, you can define the database and BI blocksize. The bad news is, you can only define the blocksize of the database when you create it - therefore if you want to change it you need to dump your database, create a new one with the desired blocksize and load it. The good news is, you can change the blocksize of the BI every time you truncate it - the database just needs to be offline for that.
There is a rule of thumb for the blocksizes - the bigger the better. Particularly the blocksizes should be equal or bigger than the file system blocksizes. If they are smaller you will have a serious performance penalty. Therefore the best settings for the database blocksize is 8K and 16 for the BI respectively (both are the maximum values).
Heavy Regards, RealHeavyDude.