Every website has a different image size for the Page Header Image, but this is customizable (just customize the "dimensions" field).
For example, if you're a photographer, you might want to maintain the 2:3 ratio which is 960px width by 640px height, so it doesn't cut off any of your images. But this is entirely up to you. It is vital, however, to keep the width as 960px otherwise it will break your design.
Keep in mind, the purpose of the Page Header Image is to display a couple of horizontal images and that's it. If you have a lot of images to display that are both portrait and landscape format, they should be placed in the Photo Gallery.