$("#modalIFrame").bind('load',
            function () {
                var newheight, 
                newwidth;

                newheight = this.contentWindow.document.body.scrollHeight;
                newwidth = this.contentWindow.document.body.scrollWidth;

                $(this).height(newheight);
                $(this).width(newwidth);
            }
        );

        $("#modalIFrame").attr('src', url);

Bind onload function to the iFrame and set the size of the iFrame to the iFrames loaded content.
In the example jQuery is used.

Resize iFrame to its content

resize

JavaScript