// retrieves url with HEAD. returns content length,
// or -1 in case of error.
static long getURLSizeByHEAD(String url) {
  url = absoluteURL(url);
  HttpURLConnection conn = null;
  InputStream in = null;
  try {
    conn = (HttpURLConnection) new URL(url).openConnection();
    conn.setRequestMethod("HEAD");
    conn.setRequestProperty("X-No-Cookies", "1");
    in = conn.getInputStream();
    return conn.getContentLength();
  } catch (IOException e) {
    return -1;
  } finally {
    closeURLConnection(conn, in);
  }
}