Resizing an iframe

Hi

I have an iframe on a page containing a webinar registration form & some text. I've basically sized this as 100 width & 1800px height.

After a visitor submits the form, however, all they see is the bottom of the page in the iframe, because the thank-you text is back at the top of the iframe.

My question then: is it possible using CSS, to make the iframe resize it's height after submission, so the visitor sees the thank you message at the top of the page. i.e. can CSS make an iframe resize when the content changes?

I hope that makes sense!

Thanks

Gerard