Unless they have something to prevent fullscreen, the request fullscreen action should work (I think they don't have anything to prevent this, that would be foolish for them)
As for the resolution, I think some people use 640x480 and 800x600, but as I said before, due to how C2 works (at least if the graphic quality is on high, which is the default option), just set the window size to a smaller value and zoom out should do the trick just fine (unless you are playing with zoom in and out in your game itself, in that case more adaptation would be needed)
I think setting the window size to 800x450 and set the layout scale to 0.625 shouldldo a quick fix, but try it out before to verify if that works well in you case (and provide a fullscreen button so people can enjoy it fullscreened )