I've found that for portrait orientations, the resolutions 640 x 960 or 320x480(fairly small, not good for upscaling) with fullscreen browser mode set to "Scale outer" work well.
As for landscape, either 1920 x 1080 or 960 x 540 with fullscreen browser mode set to "Letterbox scale" works very nicely for both desktop and mobile. This option results in a bigger game screen for desktop players.
Mostly, it depends on your target mobile platform.