SOURCE

function setIframeHeight(iframe) {
  if (iframe) {
    var iframeWin = iframe.contentWindow || iframe.contentDocument.parentWindow;
    if (iframeWin.document.body) {
      iframe.height = iframeWin.document.documentElement.scrollHeight || iframeWin.document.body.scrollHeight;
    }
  }
};

window.onload = function() {
  setIframeHeight(document.getElementById('external-frame'));
};