To dynamically resize an iframe to it's contents, set the height of the iframe to the scroll height.

<iframe frameborder="0" height="0" 
id=iframe1
marginheight="0" marginwidth="0" 
scrolling="no" width="100%"
onload="this.style.height=iframe1.document.body.scrollHeight + 5;"
src="/file.htm" >
<iframe>

Here is a function to write and dynamically resize an iframe to it's contents.

function br_page_Iframe(src, iclass, istyle, id) {
    if (!id) id = 'br_iframe_' + this.iframe_count++;
    var onload = ' onload="this.style.height=' + id + '.document.body.scrollHeight + 5;"';
    id = ' id="' + id + '"';
    if (!iclass) iclass = '';
    else iclass = ' class="' + iclass + '"';
    if (!istyle) istyle = '';
    else istyle = ' style="' + istyle;
    var out = '<iframe frameborder="0" height="0" marginheight="0" marginwidth="0" scrolling="no" width="100%"'
      + id
      + onload
      + iclass
      + istyle
      + ' src="' + src + '"'
      + '></iframe>';
    document.write(out);
}

If your page and the page contained inside the iframe are on different domains, to allow your function to execute, add this code to the page contained in the iframe, where domainoriginal is the domain name of the page containing the iframe.

<script type="text/javascript">
document.domain=domainoriginal;
</script>