It's a nice start, but it feels like it's missing something. The chara looks too seperate
* The moon doesn't match the scan; it's too textured, whereas the scan is smooth. A lot of wallers fix this
type of problem by adding clouds. Clouds not only soften the bg, but fill in some space.
* The stars look too regular. I'm not sure at this resolution, but it looks like a pure noise filter. The
cross-like stars are also too regular. I suggest downloading some more brushes and alternating them, You may also want
to try another tutorial for the bg.
* I think a well-done title would give the wall more weight and structure.
* btw: Screen size shouldn't be too much of a prob when making walls. I make 1600x1200 walls on a 1280x800
(that's widescreen, too) laptop. 1600x1200 is the best resolution for walls now, and the one you'll see most
often from long-time wallers here at MT.
You can use Quicktime to get a peak at Photoshop files while working on them to see what the whole image looks like, or
you can just zoom out in Photoshop.
Hope this helps. :)