Header image on home/frontpage

I am trying to add a background image to the header on the home/frontpage. When I go to Appearance | customize | header image it states "Your theme works best with an image with a header size of 1680 × 720 pixels — you’ll be able to crop your image once you upload it for a perfect fit." I uploaded this size of image but it was to large on the height. How do I determine the correct size of image for the header on the home/frontpage?