Meta box to add the Full Width Class to post body_class

add-full-width-option.php

// Add custom meta box
function add_full_width_meta_box() {
    $post_types = array('post', 'page'); // Add more post types if needed
    foreach ($post_types as $post_type) {
        add_meta_box(
            'full-width-meta-box',
            'Full Width',
            'full_width_meta_box_callback',
            $post_type,
            'side',
            'default'
        );
    }
}
add_action('add_meta_boxes', 'add_full_width_meta_box');

// Meta box callback function
function full_width_meta_box_callback($post) {
    $full_width = get_post_meta($post->ID, '_is_full_width', true);
    ?>
    <label>
        <input type="checkbox" name="is_full_width" <?php checked($full_width, 'on'); ?> />
        Mark as Full Width
    </label>
    <?php
}

// Save meta box data
function save_full_width_meta_data($post_id) {
    if (array_key_exists('is_full_width', $_POST)) {
        update_post_meta($post_id, '_is_full_width', 'on');
    } else {
        delete_post_meta($post_id, '_is_full_width');
    }
}
add_action('save_post', 'save_full_width_meta_data');

// Modify body class
function add_full_width_body_class($classes) {
    if (is_singular(array('post', 'page'))) {
        $full_width = get_post_meta(get_the_ID(), '_is_full_width', true);
        if ($full_width === 'on') {
            $classes[] = 'is-full-width';
        }
    }
    return $classes;
}
add_filter('body_class', 'add_full_width_body_class');

This PHP Snippet adds a custom meta check box to posts and pages, allowing the user to add the is-full-width class to the posts body class. This can be used in non GP themes to overcome container widths on an individual post basis.