<script type="text/javascript">
<!--

 var viewportwidth;
 var viewportheight;
 
 // the more standards compliant browsers (mozilla/netscape/opera/IE7) use window.innerWidth and window.innerHeight
 
 if (typeof window.innerWidth != 'undefined')
 {
      viewportwidth = window.innerWidth,
      viewportheight = window.innerHeight
 }
 
// IE6 in standards compliant mode (i.e. with a valid doctype as the first line in the document)

 else if (typeof document.documentElement != 'undefined'
     && typeof document.documentElement.clientWidth !=
     'undefined' && document.documentElement.clientWidth != 0)
 {
       viewportwidth = document.documentElement.clientWidth,
       viewportheight = document.documentElement.clientHeight
 }
 
 // older versions of IE
 
 else
 {
       viewportwidth = document.getElementsByTagName('body')[0].clientWidth,
       viewportheight = document.getElementsByTagName('body')[0].clientHeight
 }
document.write('<p>Your viewport width is '+viewportwidth+'x'+viewportheight+'</p>');
//-->
</script>

Get browser viewport width and height

window

JavaScript