Many of us have faced the dilemna of auto-sizing an IFrame based on external web content. Auto-sizing works just fine, if both the website and content are on the same domain. But, if you are trying to auto-size content using cross-domain scripting, it is nearly impossible.
I too faced this problem, but using an example I found here: http://scvdotnet.org/index.php?url=archives/8-Autofitting-iFrame.html, I was able to make this work. Please note, the example only works if you have access to both the website where the IFrame is hosted, and the website where the content is hosted. I.e. this script will not work if you are trying to auto-size an IFrame for content you do not host or have access to.