Needed auto resized iframe for my own content, and after few hours this is a working result:

Here is the code for the iframe with id="ifrm"

Code:
function autoIframe(){ 
	var x = 0;
	
	///////////// reset iframe size for Opera //////////////
	if(navigator.userAgent.indexOf("Opera")!=-1){
	this.document.body.scrollHeight = this.document.body.offsetHeight
	}
	/////////////////////IE, Opera///////////////////////////////
	var y = this.document.body.scrollHeight;
	while (x < y){
		x+=1;
	}
	parent.document.getElementById("ifrm").style.height = x
	
	///////////////////// Firefox //////////////////////////////
	if(navigator.userAgent.indexOf("Firefox")!=-1){
	parent.ifrm.frameElement.height = this.document.body.scrollHeight+5;
	}
}
body tag of the iframe content should look like this:

Code:
<body onload="autoIframe()">
Tested with Opera 9.0, IE 7.0, Firefox 2.0

Cheers!