Please notice that your top function is not inside javascript tags and your last function does not have a close tag.
Your zoomLetter(mysize) function does not work even when inserted corectly. Haven't tried to trouble shoot it cause it don't make sense when you only want to store one of two values. So use this:
Code:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<title>New document</title>
<script type="text/javascript">
function zoomLetter(mysize)
{
document.cookie = "magnif = "+mysize;
}
</script>
</head>
<body>
<input type="button" value="increase" onclick="zoomLetter(1)">
<input type="button" value="decrease" onclick="zoomLetter(-1)">
</body>
</html>