Information that was passed on to me...
As for landscape, either 1920 x 1080 or 540 x 960 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.
Portrait: 640 x 960 or 320 x 480
Landscape: 1920 x 1080 or 960 x 540 (half size)