I used the Lusid plans to build my cab.
Originally, I hinged the front and back, but after some use, I decided I did not like the look of the hinge on the front (never saw a real Arcade cabinet like that) and decided to screw the front panel it down.
I access the computer from the back hinge now. No problems at all. I put my cab on caster wheels and just wheel it forward from the wall when I need to access it. If you put your cab on hinged, shorten it by 2 - 3 inches.
I can power the unit on and off through the coin door when I need to get in that way. My computer will not power on when I turn on the main cabinet power. Something with the BIOS.
As far as the dimensions go, I would just leave it at the current size in the plans. Who knows, maybe someday you will inherit a larger monitor.