Usage:
var dim = Position.GetWindowSize();
dim[0] is the window width
dim[1] is the window height

window can be specified, useful if you work with frames..

Position.GetWindowSize = function(w) {
        w = w ? w : window;
        var width = w.innerWidth || (w.document.documentElement.clientWidth || w.document.body.clientWidth);
        var height = w.innerHeight || (w.document.documentElement.clientHeight || w.document.body.clientHeight);
        return [width, height]
}

// various functions related to viewable browser dimensions and scroll offset

putCenter: function(item) {
  item = $(item);
  var xy = item.getDimensions();
  var win = this.windowDimensions();
  var scrol = this.scrollOffset();
  item.style.left = (win[0] / 2) + scrol[0] - (xy.width / 2) + "px";
  item.style.top = (win[1] / 2) + scrol[1] - (xy.height / 2) + "px";
},
fullScreen: function(item) {
  item = $(item);
  var win = this.windowDimensions();
  var scrol = this.scrollOffset();
  item.style.height = scrol[1] + win[1] + "px";
},
windowDimensions: function() {
  var x, y;
  if (self.innerHeight) {
    // all except Explorer
    x = self.innerWidth;
    y = self.innerHeight;
  } else if (document.documentElement && document.documentElement.clientHeight) {
    // Explorer 6 Strict Mode
    x = document.documentElement.clientWidth;
    y = document.documentElement.clientHeight;
  } else if (document.body) {
    // other Explorers
    x = document.body.clientWidth;
    y = document.body.clientHeight;
  }
  if (!x) x = 0;
  if (!y) y = 0;
  arrayWindowSize = new Array(x,y);
  return arrayWindowSize;
},
scrollOffset: function() {
  var x, y;
  if (self.pageYOffset) {
    // all except Explorer
    x = self.pageXOffset;
    y = self.pageYOffset;
  } else if (document.documentElement && document.documentElement.scrollTop) {
    // Explorer 6 Strict
    x = document.documentElement.scrollLeft;
    y = document.documentElement.scrollTop;
  } else if (document.body) {
    // all other Explorers
    x = document.body.scrollLeft;
    y = document.body.scrollTop;
  }
  if (!x) x = 0;
  if (!y) y = 0;
  arrayScrollOffset = new Array(x,y);
  return arrayScrollOffset;
}