Screenheight and width ou WindowWidth and Windowheight ?
If your app is full screen there wouldn't be much difference, but screen width and height are probably the better options to use.
System:Compare two values -> Browser.ExecJS("navigator.platform.toLowerCase()") = "iphone"
System:Compare two values -> Browser.ScreenWidth = 1125
System:Compare two values -> Browser.ScreenHeight = 2436
You might want to test what values are returned for Browser.ScreenWidth and Browser.ScreenHeight, as they might change based on orientation, for which you will need to take account.