¿Algo Así? Código PHP:
<html>
<head>
<title>Untitled</title>
</head>
<body style="font:normal 12px/12px verdana">
<select name="sizer" onchange="document.getElementsByTagName('BODY')[0].style.fontSize=this.value+'px'" size="1">
<option value="9">enano</option>
<option value="10">pequeño</option>
<option value="12" SELECTED>normal</option>
<option value="20">grande</option>
<option value="26">enorme</option></select><br>
hola hola hola hola hola hola hola hola hola hola hola
</body>
</html>