function setIframeHeight(iframe)
  {
    if (document.getElementById)
    {
        if (iframe && !window.opera)
        {
            if (iframe.contentDocument && iframe.contentDocument.body.offsetHeight)
                iframe.height = iframe.contentDocument.body.offsetHeight;
            else if(iframe.Document && iframe.Document.body.scrollHeight)
                iframe.height = iframe.Document.body.scrollHeight;
        }
    }
  }

<iframe id="ifrm" width="100%" frameborder="0" scrolling="no" onload="setIframeHeight(this)" src="main.aspx"></iframe>