That's just... ugh. For one, document.write is sorely outdated. Second, what if JS is unsupported or disabled? Third, the screen size is not the viewport size.
I suspect the page design is flawed; the code you posted has what's called a smell. A better design would be to give whatever is to the left or right of the content a rigid width, using (e.g.) ems. For example:
HTML Code:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
<style type="text/css">
.menu, .menu li {
list-style-type: none;
margin: 0;
padding: 0;
}
.nav {
padding: 1em;
width: 10em;
float: left;
background: #CCC;
}
#Content {
background: #CCF;
padding: 1em 1em 1em 13em;
}
</style>
</head>
<body>
<ul class="nav menu">
<li>foo</li>
<li>bar</li>
<li>baz</li>
<li>bam</li>
<li>bug-AWWK!</li>
<li>qux</li>
<li>quux</li>
</ul>
<div id="Content">
<p>Lorem ipsum dolor sit amet, consectetuer adipiscing elit. Donec fermentum orci nec felis. Sed
sollicitudin diam id sapien. Ut libero. Vestibulum quam libero, malesuada et, ornare id, aliquet id, tellus.
Nullam dapibus viverra quam. Vestibulum sit amet nunc vel justo dictum pharetra. Proin eleifend mi eget
massa. Pellentesque feugiat sapien a ante. Duis imperdiet venenatis purus. Praesent auctor erat eu tortor.
Nulla facilisi. Nunc felis sem, ornare quis, blandit sed, elementum at, lorem. Integer hendrerit, dolor nec
mollis scelerisque, nisi libero posuere odio, et vulputate magna sem in eros.</p>
<p>Sed quam neque, hendrerit vel, faucibus nec, egestas vitae, diam. Proin vel dui non mauris dapibus
malesuada. Cras tempus urna nec enim. Maecenas neque sapien, mollis elementum, accumsan eu,
fringilla in, velit. Sed lorem. Integer sed est. Suspendisse ac odio vitae libero vulputate aliquet. Donec
pulvinar vestibulum felis. Vestibulum viverra posuere risus. Curabitur nisl magna, pellentesque in,
malesuada vitae, sodales eget, lacus.</p>
</div>
</body>
</html>
The effect is similar to what you post, but is much more fluid and works in broader circumstances.