mpi/total_size_MB is a SAP Parameter attribute which is used to control Total memory size in MB used for 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 is transferred between the Internet Communication Manager (ICM) and
the SAP work processes using memory pipes (MPI). These are pipes based
on shared memory. Parameter mpi/total_size_MB specifies the total size
of the shared memory area used for the MPIs in megabytes.
See SAP Parameter Documentation for full SAP documentation for this profile parameter.