Position.GetWindowSize = function(w) {
// Usage:
// var dim = Position.GetWindowSize();
// dim.width is the window width
// dim.height is the window height
  var width, height;
  w = w ? w : window;
  this.width = w.innerWidth || (w.document.documentElement.clientWidth || w.document.body.clientWidth);
  this.height = w.innerHeight || (w.document.documentElement.clientHeight || w.document.body.clientHeight);

  return this;
}