For the MAME side of the question - I use a 17-inch monitor and run all MAME games at
-resolution 800x600
-hwstretch 1
-keepaspect 1
You need a vidcard that supports hwstretch, but anything in the last 5 years or so should. Those last too settings should get you full screen in most games. For vector games, higher resolution is better, and on that large a screen, lower resolution might look more "accurate" for raster games.