em/initial_size_MB is a SAP Parameter attribute which is used to control Initial size of extended memory pool 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 .
Note the operating system dependency for Microsoft Windows.
The value specifies the initial size of the memory area that SAP Memory
Management manages as extended memory. The memory requirements for
interactive users' programs are met from this area. This memory type can
be assigned very quickly to changing work processes by mapping and
therefore allows very fast context switches.
Aim
Ideally, this pool should be large enough to contain the total of all
user contexts, so that context switching for all users could be
performed at optimal speed. You can restrict the maximum usage of an
individual user with the parameter
See SAP Parameter Documentation for full SAP documentation for this profile parameter.