If you'd like your header to expand across the form, we recommend making the width around 740 pixels. Of course, if you have a logo or smaller image, the width can be as small as you like. The image will be automatically centered. The height of your image can be whatever you'd like.
Looking for a tool to edit your image? Check out Pixlr, a great (and free) online image editing tool that looks a whole lot like PhotoShop.