How to change header image size in custom themes

Question

If you’re using Twenty Eleven, just add this to your functions.php file and you should be good to go:

remove_filter( 'HEADER_IMAGE_WIDTH', 'twentyeleven_header_image_width' );
remove_filter( 'HEADER_IMAGE_HEIGHT', 'twentyeleven_header_image_height' );
define( 'HEADER_IMAGE_WIDTH', apply_filters( 'child_header_image_width', 1700) );
define( 'HEADER_IMAGE_HEIGHT', apply_filters( 'child_header_image_height', 300) );