If your doing it in Construct2 then it only needs to be as big as what looks good in a browser, from what I understand, even the Windows port still uses the Chrome browser workings.
My space shooter (in my sig) was done in like 800x400 and it sizes nicely in a browser windows at full screen. It really depends on how awesome your sprite images are and at what resolution everything scales right for you.
But what about a desktop game, exported with node-webkit?
It doesn't need a "choose your resolution" option ingame?