Paste into the bottom of your code (needs jquery)

<div class="width">
    <script type="text/javascript">
        $(window).resize(function(e){
            var width = $(window).width();
            $(".width").html(width);
        });

        $(document).ready(function(e){
            var width = $(window).width();
            $(".width").html(width);
                    });
    </script>
</div>

Style with:

.width {
    position: fixed;
    background: #dd4400;
    color: white;
    height: 2em;
    line-height: 2em;
    width: 70px;
    text-align: center;
    top: 40px;
    left: 48%;
    font-weight: bold;
}