function resizeIframe() {
    var height = document.documentElement.clientHeight;
    height -= document.getElementById('mp').offsetTop;
    
    // not sure how to get this dynamically
    height -= 20; /* whatever you set your body bottom margin/padding to be */
    
    document.getElementById('mp').style.height = height +"px";
    
};
window.onresize = resizeIframe;