SNIP Extension that lets you choose a certain post thumbnail size

snip-post-thumbnail-sizes.php

<?php
/*
Plugin Name: SNIP: Post Thumbnail Sizes
Version: 0.1.0
*/

if ( ! defined('ABSPATH')) {
    exit;
} // Exit if accessed directly

/**
 *
 * PHP Version check.
 *
 */
if (version_compare(PHP_VERSION, '7.0', '<')) {
    add_action('admin_notices', 'snip_post_thumbnail_old_php_notice');

    function snip_post_thumbnail_old_php_notice()
    {

        printf(
            '<div class="notice error"><p>%s</p></div>',
            sprintf(
                __('Hey mate! Sorry for interrupting you. It seem\'s that you\'re using an old PHP version (your current version is %s). You should upgrade to at least 7.0 or higher in order to use the this plugin. Thank you!',
                    'snip-post-thumbnails'),
                esc_html(PHP_VERSION)
            )
        );
    }

    # sorry. The plugin will not work with an old PHP version.
    return;
}


add_filter('wpbuddy/rich_snippets/fields/internal_subselect/values', 'snip_post_thumbnails_subselects');

/**
 * Adds new field to the Global Snippets select boxes.
 *
 * @param array $values
 *
 * @return array
 * @since 0.1.0
 *
 */
function snip_post_thumbnails_subselects($values)
{

    $values['http://schema.org/URL'][] = array(
        'id'     => 'textfield_current_post_thumbnail_url',
        'label'  => esc_html_x('Post: Thumbnail URL (size)', 'subselect field', 'snip-post-thumbnails'),
        'method' => 'snip_post_thumbnail_size_url',
    );

    return $values;
}


/**
 * Returns the post thumbnail URL according to the size name.
 *
 * @param                                     $val
 * @param \wpbuddy\rich_snippets\Rich_Snippet $rich_snippet
 * @param array                               $meta_info
 *
 * @return string
 * @since 0.1.0
 *
 */
function snip_post_thumbnail_size_url($val, \wpbuddy\rich_snippets\Rich_Snippet $rich_snippet, array $meta_info)
{

    $src = wp_get_attachment_image_src(
        get_post_thumbnail_id($meta_info['current_post_id']),
        $val
    );

    if (isset($src[0])) {
        return $src[0];
    }

    return '';
}

This is an extension to SNIP, my Structured Data WordPress Plugin.
You can find the documentation to this on the following page: https://rich-snippets.io/use-post-thumbnail-sizes-for-images/