thats a very common size, search around this site and you'll find a bunch...anything that big or bigger will fit...with a bigger wallpaper, try setting it to be centered or tiled to get a different angle, or just resize the image yourself in photoshop or the like...
Windows should resize wallpapers for you when you apply them...or you can just resize it yourself in Paint by cutting off the edges (shrinking it in paint will lead to distortion)
