lucy24 - 5:54 am on Mar 7, 2012 (gmt 0)
You have to assign the size to the header itself, not to its background. By default, the background will use its own size-- that is, the natural size of the image. And because it's background rather than foreground, it will be cropped if it's too big.
Not sure what you mean by the second question. How big is the image? What's it a picture of? If it's meant to repeat, it should be made as small as possible: generally just one iteration each way.