DB_BLOCK_SIZE

You must set this parameter for every instance, and multiple instances must have the same value. 

Caution:

Set this parameter at the time of database creation.  Do not alter it afterward. 

DB_BLOCK_SIZE specifies (in bytes) the size of Oracle database blocks.  Typical values are 4096 and 8192.  The value of this parameter must be a multiple of the physical block size at the device level. 

The value for DB_BLOCK_SIZE in effect at the time you create the database determines the size of the blocks.  The value must remain set to its initial value. 

For Real Application Clusters, this parameter affects the maximum value of the FREELISTS storage parameter for tables and indexes.  Oracle uses one database block for each freelist group.  Decision support system (DSS) and data warehouse database environments tend to benefit from larger block size values.