Try something like this, you can adjust the values (this code will create an iframe across the whole page)
<!DOCTYPE html>
<style>
*{margin:0;padding:0}
html, body {height:100%;width:100%;overflow:hidden}
table {height:100%;width:100%;table-layout:static;border-collapse:collapse}
iframe {height:100%;width:100%}
</style>
</head>
<table>
<iframe src="http://mywebsite/somepage.html" frameborder="0"></td></tr>
</table>
</body>
</html>