The exact size of the monitor is the exact same size as every other 19" monitor in the world. The resolution the game is running is totally independent of monitor size. The two numbers are not even related. Robotron used a standard resolution monitor and the game hardware would drive any standard resolution monitor regardless of size.
With that said, different game hardware would fill up a standard res monitor differently. Those of us who have had large collections of game boards are quite familiar with having to readjust the monitor when changing boards.
So, basically your problem is not related at all to the size of your display or the size of the original display. It likely has more to do with your TV-out hardware.
Does anyone know what the exact measurement in inches of the Monitor for Robotron, I want the length and width not just the diagonal, which i think is 19