Did you remember setting the height of the html and body tags in your CSS? This is generally how I’ve gotten DIVs to extend to full height:
<html>
<head>
<style type="text/css">
html,body { height: 100%; margin: 0px; padding: 0px; }
#full { background: #0f0; height: 100% }
</style>
</head>
<body>
<div id="full">
</div>
</body>
</html>