A simple script that lets you know the window size on load and resize.
// JQUERY window size generator
$('body').append("<div id='window__width' style='background: black; color: white; font-weight: bold; padding: .4em; position: fixed; left: 20px; top: 20px; z-index: 9000;'>START</div>");
$('#window__width').text($(window).width());
$(window).resize(function() {
width = $(window).width();
$('#window__width').text(width);
});