javascript

<iframe id="iframe" src="https://www.example.com"></iframe>
<script>
    var iframe = document.getElementById("iframe");
    iframe.onload = function() {
        this.style.height = this.contentWindow.document.body.scrollHeight + 'px';
    };
</script>

jQuery
<iframe id="iframe" src="https://www.example.com"></iframe>
<script>
    $("#iframe").on("load", function() {
        $(this).height($(this).contents().find("body").height());
    });
</script>