If you're talking about mame's game screen, you can
make a lay file with the boarders you want, and set mame to use the layout.
Not sure the best way to do this for you case, though. You might try in the lay generator page, put any game name, put any file, set "bezel size" to your LCD's resolution, set "bezel window" to slightly smaller than your monitor res, set "Screen Aspect ratio" to 3:4 (this is for the
game's aspect ratio, NOT your monitor's aspect ratio), and set the boarder % to whatever you want. Pre recalculate to see a preview. Generate and copy paste to to a file.
I'm not fluent in the lay files, so you might want to look at mameDev wiki's
artwork and
lay file pages for more info.
OTOH, if you also want to change the size outside of mame...
