You do not have to set a value for the DB_CACHE_SIZE initialization parameter if you use automatic memory management. The setting for the DB_CACHE_SIZE parameter determines the size of the buffer cache. This buffer cache stores metadata blocks. The default value for this parameter is suitable for most environments.
See Also:
Oracle Database Administrator's Guide for more information about setting the DB_CACHE_SIZE initialization parameter
Oracle Database Performance Tuning Guide for more information about memory configuration
Oracle Database Reference for more information about the DB_CACHE_SIZE parameter