.ri_iframe {
    position: relative;
    width: 100%;
    padding-top: 56.25%; /* this is the % to 16:9 aspect ratio */
}

.ri_iframe iframe {
    position: absolute;
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;
    width: 85%; /* you can set width and height as you wish, but they must be equals */
    height: 85%;
    margin: 0 auto;
}