UUGGH. HELP.

OK, so I'm at my friend's house right now. (The URL to her blog is at the end of this post...) So I'm trying to edit her blog for her, but she wants a background that's different from what I have. She wants one that will suit the screen. However I have no idea what size I should make it so that it will fit the screen. Every time I try editing it so the size would fit the screen, it doesn't work. May someone help me?
>.< Oh shoot her laptop is about to die... anyways, right now she's beside me singing ET by Katy Perry on super fast mode, like srsly I can't even understand what she's saying she's going so fast. This is what happens when you speed up the song. ._. AND it's on repeat... oh, nvm the song just changed and she ran upstairs squealing to get the laptop cord to try and save it. Hmmm I should probably hurry up... anyways, plz help. D: And for some reason the music on my blog doesn't work on hers. I'm guessing it's because she has Google Chrome. Since the music works on IE, I'm guessing that it doesn't work on other (what do you call them?) ones, like Opera or FireFox, so on and so forth. I have no idea how to fix that. UGH. I guess that's it, so please, someone gimme me the correct size. Dx I also tried putting the full size proper backgrounds made for blogs into a new gadget, with the code you copied and pasted from the site with the background. But for some reason it didn't like the type of template (or wutever it's called) that my friend had set to her blog and nothing else really works... we're going to keep working on it (Or, well I am, she's now singing to a Kelly Clarkson in super fast mode), and lemme know if you get the size of the screen or wutever. (Like how the background of your desktop is set to a certain size, like that.) So, just an update.

PS-I think I'm just going to try a different template... that should work. :/

My friend's blog... (the url...)

http://nellanablog.blogspot.com/