Personally, I'm on a 1680x1050 monitor.
But, rather than absolute resolution, shouldn't the problem be limited to which
aspect ratio is the most common?
Whichever resolution you pick for the game engine output to be, won't the monitor's firmware scale the game's output to fill itself? In which case, once you pick a resolution, you would've made the game playable with no distortions on any monitor with the same aspect ratio as the resolution you've picked. Once that is sorted, you can include an option to add pillarboxing or letterboxing bars in postprocessing (so your game engine would still be outputting the fixed resolution).
Eg. Suppose you find out that 16:10 is the most common aspect ratio. If this were the case, it would be sensible to pick a resolution with a 16:10 AR.
Say you pick 1280x800 as your fixed resolution for your game engine output. This should make the game playable with no distortion on any monitor with a 16:10 spect ratio. If the game is played on a 1680x1050 monitor, the game engine may as well output 1280x800-sized frames, but to the best of my knowledge this output will be scaled by the monitor to fill the entirety of the monitor's surface, rather then windowboxing it in the middle of the screen!
That said, if you add a "Keep 4:3 aspect ratio" in game option, should the player own a 4:3 monitor and enable the option, the game engine would still be outputting a 1280x800 frame, to which a padding of 80-pixel thick bars above and below the image would be added in postprocessing, creating a 1280x960 frame with a 4:3 aspect ratio, which would then be scaled up by the screen, resulting in a letterboxed image with correct proportions.
Does it make any sense? I've not a professional, so what I said may make sense in theory but be inapplicable in practice. In which case, sorry to waste your time.
