rsdb/ntab/ftabsize is a SAP Parameter attribute which is used to control data area size for field description 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 field description buffer for storing a
description of an SAP table that is accessed at runtime.
See SAP Parameter Documentation for full SAP documentation for this profile parameter.