How to make Twitter background fullscreen?

[Rinnosuke]

I've uploaded some images of different sizes, but I can't seem to find one that fits the entire screen. They only allow me to move it to the right, center, and left. I tiled it, though, I don't want it to look like that. How big of an image can I change it to for it to be covering the whole screen?
Tell me the size of the image (example: 800x600) and I will re-size the image. I appreciate it. Thanks.