Got a fix for you guys! I had the EXACT same issue. Screen too large using "fullscreen_window".
This is what I did to fix it. The machine I am using to run my cab is an old Microsoft Surface Pro 2 (works like a champ BTW) and the display mode was set to "Duplicate Windows"....Within your Display Settings, don't duplicate the window, but only show the display you are using. In my case that was my LCD screen on the cab. Doing this made Singe correct itself and the guns were good to go! So to recap.
- Don't Duplicate Displays, Show Desktop on only on your main display
- Add the "fullscreen_window" code
- Praise the arcade gods!
this is the screen in Windows 7

Good Luck!