Get WP post featured image URL for defined image size.

featured_image.php

<?php

function featured_image($postid, $size) {

    $image = get_post_thumbnail_id($postid);
    $image_url = $image ? wp_get_attachment_image_src($image, $size, true) : '';
    $image_url = is_array($image_url) ? $image_url[0] : null;

    return $image_url;
    
}