intenta asi, saludos
Código HTML:
<?xml version="1.0" encoding="iso-8859-1"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<style type="text/css">
* {
border: 0px;
margin: 0px;
padding: 0px;
}
.miMarco {
width: 100%;
height: 100%;
border: 0px;
}
#tabla {
width: 100%;
height: 100%;
border: 0px;
}
</style>
<script type="text/javascript">
<!--
function redimensionar() {
if (navigator.appName == 'Microsoft Internet Explorer') {
document.getElementById('tabla').style.height = '100%';
} else {
document.getElementById('tabla').style.height = window.innerHeight + 'px';
}
}
//-->
</script>
<title>Asun</title>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
</head>
<body onload="javascript:redimensionar()">
<table width='100%' height='75%' border='0' bgcolor='#47b7f6'>
<tr>
<td>
<table width='100%' border='0'>
<td>Asunto:</td>
<td>Asunto1</td>
</tr>
</table>
</td>
<td align='right'> Lista
</td>
</table>
<table id="tabla">
<tr>
<td style="width: 100%; height: 100%;">
<iframe class="miMarco" src="http://www.forosdelweb.com"></iframe>
</td>
</tr>
</table>
</body>
</html>