neilgee/header-css.css

## header-css.css
.site-header {
  background-size: cover !important;
  min-height: 200px;
  margin-bottom: 20px;
}

## header.php
<?php
/**
 * The template for displaying the header
 *
 * Displays all of the head element and everything up until the "site-content" div.
 *
 * @package WordPress
 * @subpackage Twenty_Sixteen
 * @since Twenty Sixteen 1.0
 */

?><!DOCTYPE html>
<html <?php language_attributes(); ?> class="no-js">
<head>
	<meta charset="<?php bloginfo( 'charset' ); ?>">
	<meta name="viewport" content="width=device-width, initial-scale=1">
	<link rel="profile" href="http://gmpg.org/xfn/11">
	<?php if ( is_singular() && pings_open( get_queried_object() ) ) : ?>
	<link rel="pingback" href="<?php bloginfo( 'pingback_url' ); ?>">
	<?php endif; ?>
	<?php wp_head(); ?>
</head>

<body <?php body_class(); ?>>
<div id="page" class="site">
	<div class="site-inner">
		<a class="skip-link screen-reader-text" href="#content"><?php _e( 'Skip to content', 'twentysixteen' ); ?></a>

		<header id="masthead" class="site-header" role="banner" style="background:url(<?php echo get_header_image()?>) center no-repeat;">
			<div class="site-header-main">
				<div class="site-branding">
					<?php if ( is_front_page() && is_home() ) : ?>
						<h1 class="site-title"><a href="<?php echo esc_url( home_url( '/' ) ); ?>" rel="home"><?php bloginfo( 'name' ); ?></a></h1>
					<?php else : ?>
						<p class="site-title"><a href="<?php echo esc_url( home_url( '/' ) ); ?>" rel="home"><?php bloginfo( 'name' ); ?></a></p>
					<?php endif;

					$description = get_bloginfo( 'description', 'display' );
					if ( $description || is_customize_preview() ) : ?>
						<p class="site-description"><?php echo $description; ?></p>
					<?php endif; ?>
				</div><!-- .site-branding -->

				<?php if ( has_nav_menu( 'primary' ) || has_nav_menu( 'social' ) ) : ?>
					<button id="menu-toggle" class="menu-toggle"><?php _e( 'Menu', 'twentysixteen' ); ?></button>

					<div id="site-header-menu" class="site-header-menu">
						<?php if ( has_nav_menu( 'primary' ) ) : ?>
							<nav id="site-navigation" class="main-navigation" role="navigation" aria-label="<?php _e( 'Primary Menu', 'twentysixteen' ); ?>">
								<?php
									wp_nav_menu( array(
										'theme_location' => 'primary',
										'menu_class'     => 'primary-menu',
									 ) );
								?>
							</nav><!-- .main-navigation -->
						<?php endif; ?>

						<?php if ( has_nav_menu( 'social' ) ) : ?>
							<nav id="social-navigation" class="social-navigation" role="navigation" aria-label="<?php _e( 'Social Links Menu', 'twentysixteen' ); ?>">
								<?php
									wp_nav_menu( array(
										'theme_location' => 'social',
										'menu_class'     => 'social-links-menu',
										'depth'          => 1,
										'link_before'    => '<span class="screen-reader-text">',
										'link_after'     => '</span>',
									) );
								?>
							</nav><!-- .social-navigation -->
						<?php endif; ?>
					</div><!-- .site-header-menu -->
				<?php endif; ?>
			</div><!-- .site-header-main -->

		</header><!-- .site-header -->

		<div id="content" class="site-content">

## ts-added-css.css
/* add to end of style.css */

/* Make Twenty Sixteen full width header */

.site-inner {
	max-width: none;
}

.site-content,
.site-footer,
.site-header-main {
	max-width: 1320px;
	margin: 0 auto;
}

@media screen and (min-width: 44.375em) {
	.site {
	    margin: 0;
	}

	body:not(.custom-background-image):before, body:not(.custom-background-image):after {
		height: 0;
	}
}

## twenty-sixteen-header-filter.php
<?php  //<- don't add me in

add_filter( 'twentysixteen_custom_header_args', 'wpb_new_header_size');
//add your parameters in - https://codex.wordpress.org/Custom_Headers
function wpb_new_header_size($array) {
		$array = array (
		'default-text-color'     => $default_text_color,
		'width'                  => 1600,
		'height'                 => 200,
		'flex-height'            => true,
		'wp-head-callback'       => 'twentysixteen_header_style',
	);
		return $array;

}