whats the height and width to cover the xanga page?

This depends on the screen resolution. It's different for different people. The typical screen resolutions are 800x600 pixels and 1024x768 pixels. If you make a back ground image you can't insure the iamge will fill the persons whole xanga background and it might be too large for thier background too.

What I would do is make an image that is on the smaller side. Something less than 1024x768 and larger than 800x600 then center the background image in your xanga so it looks aligned on all different resolutions.

Well, if you make it too big, you're going to have a horizontal scrollbar. I think you should just make something like 950 x ----, and make it a background image, or you can just slice the images and make part of it a background, that will work too.