imo making a desktop wallpaper shouldn't be that much different than a sig (although it may end up alot more time consuming depending on how complex you want it). You should be using renders and brushes (that are over 1000px w/o losing image quality). If you have those...just zoom out of the image/canvas and work on it like a normal sig. I usually find vector brushes to be the easiest to work with because they generally come in over 1000px. But of course this may vary depending on your wallpaper size (i have a 1680x1050 wallpaper).