Actually, talking with TK, the RCS is modeled by taking the aggregate size of the 'hitboxes' defined in the data.ini for the particular aircraft.
Then, any radars looking for it, the signal strength defines how far the radar can see it.
So, most non-stealth aircraft, the range at which you pick it up on radar is directly related to it's physical size (which makes it easier in that you don't need an RCS modifier for EVERY aircraft).
For stealth aircraft, the BaseRCSModifier is basically a multiplier used to lower or raise the RCS in the game. Using the modifier allows you to tweak certain aircraft (F-18E/F as an example) to lower their RCS, or in the case of the F-117, F-22, F-35, to lower them a lot relative to their physical size.
Be advised, this only works like this for games patched to Oct 08 or later.
FC