em/stat_log_size_MB is a SAP Parameter attribute which is used to control Insert into history buffer over MB used 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 .
In the SAP memory management system, if the sizes of user contexts
exceed the specified value, they are collected for statistical purposes.
See SAP Parameter Documentation for full SAP documentation for this profile parameter.