I am trying to make html5 games to be played on the browser(not offline apps), and I am trying to support the maximum number of platforms, hence I need to know what dimension should I use for the game canvas so that it works in the most number of places.

Also is there anyway to "scale" a large game to fit in the tiny size of iphone(around 320x356px I think). By "scale" I don't mean to actually resize just the canvas, as because that can mess up the coordinate based calculations, and for a large number of objects, re-positioning based on canvas size can be a real hassle.