Firstly you could really do with a program like Paintshop Pro or Photoshop.🙂 It's really trial and error I found as to what works best, plus individual taste. I had only made small/medium banners before I attempted a wallpaper but got bored one day and so decided to have a go. I ended up making about 4 or 5 and for my first attempts I was quite pleased with the results.
It would be hard to talk someone thorugh making a wallpaper - you might be best downloading one of the above programs, and then searching for a tutorial on them, there are plenty around, and then just have a go, the more you practice, the better you will become! 😉 Size wise depends - I make mine 800x600 as that's what fits my screen.
I'm sure you'll be able to find out more by searching the net, especially if you can get hold of one of those programs I mentioned.🙂
Good luck anyways! 😀😀