vsi/cache/size_MB is a SAP Parameter attribute which is used to control Maximum size of mega bytes which should be scanned in memory 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 defines the maximum amount of data that a virus scan
should transfer to the virus scanner as an internal table in memory with
CL_VSI->SCAN_ITAB. Above this limit, a temporary file is created and
When scanning files, ensure that there is enough storage space available
in the temporary directory DIR_TEMP, and that, if there is a scanner in
on-access mode, that it is not locking this temporary file.
See SAP Parameter Documentation for full SAP documentation for this profile parameter.