What screen size setting should I use if I want my game to be on both Desktop and Mobile?

If a game designer wants his game to be for both Windows Desktop and Android mobile phones, what settings should one use?  Does one just use the Tablet Preset of 1024 x 768 or is there a custom Width and Height setting that should be used?  Finally, how does one actually go about creating two versions of the same game for both Windows Desktop and Android phones?