shortcode get thumbnail size

I think your code is very close, try to use wp_get_attachment_image_src() instead:

function thumb_medium( $atts, $content = null ) {
    // return wp_get_attachment_url( get_post_thumbnail_id( $post_id, 'medium') );
    global $post;
    $thumb_url = wp_get_attachment_image_src( get_post_thumbnail_id($post->ID), 'medium' )[0];
    if ($thumb_url) {
        return $thumb_url;
    }
}
add_shortcode("get_urlthumb", "thumb_medium");