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.
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