the monitor size could be anything, if it's been converted, it more than likely doesn't have the original monitor in it. originally it had a 19 inch.
as far as your marquee goes, size is relative depending on the printer or output... your best bet is to use a drawing program like "corel draw", where you can plug in the size you need and it automatically adjusts the printout to match perfectly. (IE: 200 pixels on one printer is smaller or bigger on another printer due to differences in resolution.)
also regarding the marquee, it's best to actually measure it once you get the cab... differences in manufacturing could make it slightly bigger or smaller than what it's supposed to be...it's bet to custom cut/make the marquee so that it fits tight with no gaps or slop...you also want it to actually fit should it be a little smaller.
good luck on your project, should you need any more help, feel free to post