The header image of your site is 1120 x 373 pixels. If you want to create a custom header in an application like Photoshop, to add your branding or other artwork, this is the size to use. You can save your image as either a jpg or png.
You can also upload a bigger image than this and then use the crop tool to select the part of the image that will show.
Watch the tutorial video:
How To Add A Header Image