After reading lots of answers here they all had the same issue with not resizing smaller when needed. I think most people are just doing a one-off resizing after the frame loads, so maybe don't care. I need to resize again anytime the window size changes. So for me, if they made the window narrow the iframe would get very tall, then when they make the window larger it should get shorter again. This wasn't happening on some browsers because the scrollHeight, clientHeight, jquery height() and any other height I could find with DOM inspectors (FireBug/Chrome Dev Tools) did not report the body or html height as being shorter after the iframe was made wider. Like the body had min-height 100% set or something.
For me the solution was to make the iframe 0 height, then check the scrollHeight, then set to that value. To avoid the scrollbar on my page jumping around, I set the height of the parent (that contains the iframe) to the iframe height to keep the total page size fixed while doing this.
I wish I had a cleaner sample, but here is the code I have:
$(element).parent().height($(element).height());
$(element).height(0);
$(element).height($(element).contents().height());
$(element).parent().height("");
element is my iframe.
The iframe has width: 100% style set and is inside a div with default styles (block).
Code is jquery, and sets the div height to the iframe height, then sets iframe to 0 height, then sets iframe to the contents height. If I remove the line that sets the iframe to 0 height, the iframe will get larger when needed, but never smaller.