Good question Keith, I honestly don't think anyone has ever asked this one...
I would like to know the answer myself. You can always try a simple process of elimination, take the wallpaper you want to use and make a few versions. We know the height, just make a few copies knocking the width down by 50 or 100 pixels at a time until you get zeroed in on the correct number.
or
Maybe someone will have an answer..