Source

<script>
    function change_size() {
        var width = parseInt($("#Width").val());
        var height = parseInt($("#Height").val());

        if(!width || isNaN(width)) {
            width = 600;
        }
        if(!height || isNaN(height)) {
            height = 400;
        }
        $("#Demo").width(width).height(height);

        // update perfect scrollbar
        $('#Demo').perfectScrollbar('update');
    }
    $(function() {
        $('#Demo').perfectScrollbar();
    });
</script>