I may have a solution for you but it could be considered "dirty".
Firstly I set your layout to same size a project (640, 480), then I set "Fullscreen in browser" to "scale".
Now on start of layout I set the scale of the layout to WindowsWidth / LayoutWidth ( if Windows was 1000 and your layout was 500 that would equall 2, so game is scaled up x2).
Added a condition inside that to decide if scale by Width or Height is greater than 1 (if not don't scale), and also a min() to decided to scale by either Width or Height.
Scale : min(floor(WindowWidth / LayoutWidth),floor(WindowHeight / LayoutHeight)) > 1 ? min(floor(WindowWidth / LayoutWidth),floor(WindowHeight / LayoutHeight)) : 1
You'll need to do this to all your layouts.https://dl.dropbox.com/u/2175584/Construct2Examples/ScreenProblems.capx