Header size

Hi! I'm using the Photofocus Pro theme, and hoping to change the height of the header. The recommended dimensions are 1920 x 1080, but I'm looking for a way to change the 1080 to a smaller px count. Is this possible? At the moment I do have the option to crop the image further when uploading, but this results in a further zoomed in image that still has the same dimensions as recommended. Thanks in advance