Unfortunately you'll probably not have a great outcome with this, purely because of how RA2 has been built. My recommendation, unfortunately, would be to stick to one of the resolutions RA2 recommends. It's hardly a game where the graphics are going to get a lot better with larger resolution, at least in my view.
Lots of RA2 just doesn't scale with resolution very well, in short. Sorry about that! Hopefully someone else might have a better solution...