How to resize iframe with real size

I've visited lots of posts but none of them gived me an answer that works. I want to resize an iframe to the parent, for example a div. The iframe tag resizes dynamically the width, but not the height. How can I dynamically resize both? If I write: <iframe src="http..." width="100%" height="400"></iframe> it always is 400 px of height. But if I put a % there too it doesn't resize properly.

And if I want to have the iframe width and height of the page it is referencing in src, how can I do it?