alchymyth on "Twenty Twelve Header size reduction"

although you seem to have fixed it already, here is a way to change the header image dimensions:

in functions.php of the child theme, add:

function twentytwelvechild_custom_header_setup() {
	$header_args = array( 'height' => 150, 'width' => 950 );
	add_theme_support( 'custom-header', $header_args );
}
add_action( 'after_setup_theme', 'twentytwelvechild_custom_header_setup' );

the padding comes from:

/* Header */
.site-header {
	padding: 24px 0;
	padding: 1.714285714rem 0;
}