PARALLEL_EXECUTION_MESSAGE_SIZE specifies the size of messages for parallel execution (formerly referred to as parallel query, PDML, Parallel Recovery, replication).
On most platforms, the default value is 2148bytes if PARALLEL_AUTOMATIC_TUNING is set to false, and 4096 bytes if PARALLEL_AUTOMATIC_TUNING is set to true. The default value is adequate for most applications. Larger values require a larger shared pool. Larger values result in better performance at the cost of higher memory use. For this reason, replication gets no benefit from increasing the size.
When PARALLEL_AUTOMATIC_TUNING is set to TRUE, message buffers are allocated out of the large pool. In this case, the default is generally higher. Note that the PARALLEL_AUTOMATIC_TUNING parameter has been deprecated.