php – Need help with a underscore theme

I’m trying to change the default custom header size; it seems embedded in the img tag.
I placed this code in my functions.php

    $defaults = array(
        // Default Header Image to display
        'default-image'         => get_template_directory_uri() . '/images/headers/default.jpg',
        // Display the header text along with the image
        'header-text'           => false,
        // Header text color default
        'default-text-color'        => '000',
        // Header image width (in pixels)
        'width'             => 3000,
        // Header image height (in pixels)
        'height'            => 250,
        // Header image random rotation default
        'random-default'        => false,
        // Enable upload of image file in admin 
        'uploads'       => false,
        // function to be called in theme head section
        'wp-head-callback'      => 'wphead_cb',
        //  function to be called in preview page head section
        'admin-head-callback'       => 'adminhead_cb',
        // function to produce preview markup in the admin screen
        'admin-preview-callback'    => 'adminpreview_cb',
        );
}
add_action( 'after_setup_theme', 'themename_custom_header_setup' );

The code is from WordPress site Documation I think.
any help would be greatly appreciated.