Here is something I just wrote for you.
Code:
<html>
<head>
<title>Text size Control Via Javascript</title>
</head>
<script language="javascript">
function ChangeFontSize()
{
var newFontSize;
var selIndex = document.frmTextSize.ddlFontSizeSelector.selectedIndex;
newFontSize = document.frmTextSize.ddlFontSizeSelector.options[selIndex].value;
document.body.style.fontSize = newFontSize;
}
</script>
<body>
<p>Test Text!Test Text!Test Text!Test Text!Test Text!Test Text!Test Text!Test Text!Test Text!</p>
<form id="frmTextSize" name="frmTextSize">
<select id="ddlFontSizeSelector" onclick="ChangeFontSize();">
<option value="10pt">Tiny</option>
<option value="12pt" selected>Small</option>
<option value="14pt">Medium</option>
<option value="16pt">Large</option>
<option value="18pt">Huge</option>
</select>
</form>
</body>
</html>
Obviously, modify it to work in your site. It works off modifying the body tags style sheet element, so any inline styles will override it, but that could be modified.
You can view it in action here.