How to change default header image dimensions in twentyfourteen child theme?

I’m creating a child theme based on twentyfourteen and wish to change the custom header image dimensions. The custom header page states:

Images should be at least 1260 pixels wide. Suggested width is 1260 pixels. Suggested height is 240 pixels.

This appears to come from defaults set in /inc/custom-header.php:

function twentyfourteen_custom_header_setup() {
    add_theme_support( 'custom-header', apply_filters( 'twentyfourteen_custom_header_args', array(
        'default-text-color'     => 'fff',
        'width'                  => 1260,
        'height'                 => 240,
        'flex-height'            => true,
        'wp-head-callback'       => 'twentyfourteen_header_style',
        'admin-head-callback'    => 'twentyfourteen_admin_header_style',
        'admin-preview-callback' => 'twentyfourteen_admin_header_image',
    ) ) );
}
add_action( 'after_setup_theme', 'twentyfourteen_custom_header_setup' );

This file is included by twentyfourteen/functions.php:

require get_template_directory() . '/inc/custom-header.php';

Because this line uses get_template_directory() and not get_stylesheet_directory(), I can’t override it with my own version of custom-header.php.

Is there a way I can change the requested dimensions in my child theme?

The is a filter to the custom header function in the twenty fourteen theme. You can use that to add your new sizes. Here is what I use to change the default size.

function wpse_custom_header_setup() {
    add_theme_support( 'custom-header', apply_filters( 'wpse_header_args', array(
        'width'                  => 1460,
        'height'                 => 220,
    ) ) );
}
add_action( 'after_setup_theme', 'wpse_custom_header_setup' );

EDIT

Also set max width of site-header found in lines 847 to 853 accordingly

.site-header {
    max-width: 1460px;
}