public function sizeUrl($size)
    {
        if (($this->base['return_format'] === 'array') && isset($this->base['value']['sizes'][$size])) {
            return $this->base['value']['sizes'][$size];
        } elseif ($this->base['return_format'] === 'id') {
            if ($src = wp_get_attachment_image_src($this->id(), $size)) {
                return $src[0];
            }
        }

        return '';
    }