es2/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 .
This parameter specifies the initial size of the background resource
where the contexts are backed up. If no more space is available on
the background resources, a new one will be created with the size of
es2/initial_size_MB / 2 MB (extend)
See SAP Parameter Documentation for full SAP documentation for this profile parameter.