You do not have to set a value for the SHARED_POOL_SIZE initialization parameter if you use automatic memory management. The setting for the SHARED_POOL_SIZE parameter determines the amount of memory required to manage the instance. The setting for this parameter is also used to determine the amount of space that is allocated for extent storage. The default value for this parameter is suitable for most environments.
Oracle Database Administrator's Guide for more information about setting the SHARED_POOL_SIZE initialization parameter
Oracle Database Performance Tuning Guide for more information about memory configuration
Oracle Database Reference for more information about the SHARED_POOL_SIZE parameter