mpi/buffer_size is a SAP Parameter attribute which is used to control Buffer size for memory pipes (MPI) information. This is available within R/3 SAP systems depending on the version and release level.
Below is the standard documentation available and a few details of the attributes values .
Data transfer within memory pipes is conducted in blocks of a set
length. Parameter mpi/buffer_size defines the size of these blocks.
See SAP Parameter Documentation for full SAP documentation for this profile parameter.