availHeight property is used to return the height of the visitor's screen.
<html>
<body>
<script type="text/javascript">
document.write("The Height of the Screen is: " + screen.availHeight);
</script>
</body>
</html>
availWidth property is used to return the width of the visitor's screen.
<html>
<body>
<script type="text/javascript">
document.write("The Width of the Screen is: " + screen.availWidth);
</script>
</body>
</html>