It varies depending on the end result you want.
For the most part, you would be looking for a image with at least 3360x1080 resolution. Since one monitor is smaller you just add the horizontal resolution to get your length, but the height is dictated by the tallest monitor (resolution wise) since the smaller one would just crop to fit.
Its not going to look right if you just span one image, one monitor will be slightly cropped.
When I had different resolution monitors with a spanned image I composite stitched two together in gimp that matched each monitor on their own. Then eventually I just have windows cycle through wallpapers on each monitor, works better.