rstr/buffer_size_kB is a SAP Parameter attribute which is used to control Size of trace internal buffer 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 SAP trace (SQL trace) saves the trace data in trace files. To
minimize the effect on system performance the data is saved in an
internal buffer, and not saved directly. Profile parameter
rstr/buffer_size_kB determines the size of this buffer.
See SAP Parameter Documentation for full SAP documentation for this profile parameter.