setInterval.aspx font size:
View Sample
<html>
<head>
<title>Abakada DHTML Demo: A Simple Clock</title>
<link rel="stylesheet" href="/shared/netdemos.css">

<script language="JavaScript">
<!--
function setClock ( ) {
   setInterval ( tick, 1000 );
}

var ratio = 4;
function tick ( ) {
   var s = Date ( );
   var t = s.substring ( 11, 19 );
   var doc_height = document.body.offsetHeight / 2;
   var doc_width = document.body.offsetWidth / 2;
   if ( ( doc_height * ratio ) > doc_width )
      doc_height = doc_width / ratio;
   theTime.innerText = t;
   theTime.style.fontSize = doc_height;
}
//-->
</script>
</head>

<body onload="setClock ( )">
<!-- #include virtual="~/shared/top.inc" -->

<div class="header"><h2>Abakada DHTML Demo: A Simple Clock</h2></div>
<hr size=1 width=92%>

<div align="center">

   <p id="msg">This example demonstrates a simple clock that adjusts the size of its readout to fit the current width and height of the document body. Resize this window to see. </p>

   <div id="theTime" style="font: bold arial; color: steelblue"></div>

</div>

<hr size=1 width=92%>
<!-- #include virtual="~/shared/viewsrc.inc" -->

</body>
</html>