scsa/shm/size is a SAP Parameter attribute which is used to control Shared Common System Area Size 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 .
The SCSA is a shared memory area that is used by basis components such
as system log, trace, and so on. scsa/shm/size configures the size of
the SCSA. Depending on the operating system, this area is created either
as a standalone shared memory area, or as area no. 5 on the SAP IPC
See SAP Parameter Documentation for full SAP documentation for this profile parameter.