rsdb/ntab/irbdsize is a SAP Parameter attribute which is used to control data area size for Initial records 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 .
Data area size for the "Initial record buffer" for storing the initial
record layout of an SAP table that is accessed at runtime.
See SAP Parameter Documentation for full SAP documentation for this profile parameter.