Add this to either the head of your document, or load as an external script, and it will add a small widget to the top left corner of your browser window showing the current width of the viewport. Make sure you load jQuery first.

viewport-width.js

$(document).ready(function() {

  var $counter = $('<p class="viewport-width-counter" />');
  $('html').append($counter);
  
  if ($('p.viewport-width-counter').length) {

    $counter.css({
      'position': 'fixed',
      'top': 0,
      'left': 0,
      'padding': 5,
      'border-radius': '0 0 3px 0',
      'background': '#999',
      'font-family': 'sans-serif',
      'margin': 0,
      'box-shadow': 'rgba(0, 0, 0, 0.6) 1px 1px 0px',
      'color': 'white',
      'z-index': 9999
    });

    var updateViewportWidthCounter = function() {
      $counter.text($(window).width());
    }

    updateViewportWidthCounter();
    $(window).on('resize', updateViewportWidthCounter);
  }
});