i made mine the biggest size possible for a laptop monitor (1680x1400 ish). i'm guessing if you have something centred on the screen it won't matter what size you make it since it will be centred no matter what size your display is.
Aken your code doesn't work for my website, when i tried it it makes my background blank.
Easy. Just ad the word: fixed to the "page background" and/or "body" main css options in your admincp style manager. For example: #050505 url(put here the url to the image you want - inside your image folder or even an external url) repeat fixed;. The important here is the fixed, the repeat word is for if your image it's too small to fill the monitor area.
I just logged on and was looking for the exact same thing, Thanks for the tip m8