i've literally holesawed a 25" bezel to swap a 25" monitor into a 27" cabinet after it failed and i needed it up and running.
the exact dimensions do not matter, only that the 5 emitters are evenly spaced across the top and bottom of your screen... and the activation lines properly strobe out the pattern the gun/game logic is looking for.
this is why this system is used in the first place... it does not matter what size screen (what cabinet) is used, be it a 27" CRT cabinet or a 50" super deluxe rear projection... they need not reprogram the game to make it work, just so long as the gun camera can "see" most of the emitters (IE be far enough away) and the LED lights strobe in the appropriate fashion...it will work.
the gun calibration in the game takes care of the rest.