Something like this.

http://i32.tinypic.com/e8n6lu.jpg

I know thats not the perfect width, because
on the music page
it's not even.
see how theirs more space beside the navbar?
I want it to be like perfect so the middle of the background is lined right next to the navbar.

so what's the width, and what's a good height for it?

The width is fine. It's the alignment of the background on the canvas that's the issue. The height depends on however large you need the background to be. Don't know why people ask these questions. You're the designer, you're supposed to decide what dimensions to use.

well how do you fix the issue?

i don't know.
it's just that every time I attempt to use
this kind of background, the image is so big
that both ends of the sides are always hidden
from the screen

and the right looks skinnier then the left...

If your screen is smaller than the width of your background, of course they won't show. To make sure the right and left are proportionate, find the center of the canvas and make a selection to the right up to wherever your marker is. Find out the width, then do the same to the left and make sure the width is the same. If they're the same, then the position of your tables is the issue. That, you can fix using a code.

How can i find the width and the right mark of the image..?
Cause all i know the width of the middle should be like 800

Is the page you linked yours? Is that the one you're trying to fix?

No but i used that profile as a example.

The page would be completely fine if it wasn't for the Status widget on the left. It is stretching the whole column and making the tables go over the borders on the background. The background isn't the problem. If you have a similar issue but you don't want to remove whatever is stretching the tables, just adjust the background accordingly. Make the center section wider until everything fits inside it.

Yeah I knew that it was stretching, but i was looking at the space between the Navbar and the background. I know it isn't much space from it, but I still want it all to be even.

There's only one bloody pixel of difference! Just move the right border over one pixel and you're sorted. It doesn't matter what width it has to be because that depends on your content.