It depends on a few things. Makesure you are running the javascript after the dom has loaded, either using a callback function for the window.load, as a triggered event from a target or placing the code at the bottom of the document which will be run immediately.
How are you applying the increase to font size?
for(var obj in document.getElementsByTagName("div")){
document.getElementsByTagName("div")[obj].style.fontSize = "16px";
}
or
document.getElementById("youeElementID").style.fontSize = "16px";
For example I have drummed up a quick html example. Please run to see the effects
<html>
<head>
<title></title>
<script type="text/javascript">
function useByTag(){
for(var i = 0; i < document.getElementsByTagName("div").length; i++){
document.getElementsByTagName("div")[i].style.fontSize = "22px";
}
}
function useByID(){
document.getElementById("Element1").style.fontSize = "18px";
}
</script>
</head>
<body>
<a href="javascript:useByTag()">Test ByTag</a>
<a href="javascript:useByID()">Test ByID</a>
<br/>
<div>Some text</div>
<div>Some text</div>
<div>Some text</div>
<div>Some text</div>
<div>Some text</div>
<div>Some text</div>
<div id="Element1">Some text</a>
</body>
</html>
