<script type="text/javascript" src="jquery/1.10.2/jquery.min.js"></script>
<script type="text/javascript">
    $(document).ready(function(){
        $("a.#G").click(function(){
            var divNode = document.getElementById("text");
                divNode.style.fontSize=Font_Size[0].GREAT+"px";
        }
        $("a.#M").click(function(){
            var divNode = document.getElementById("text");
            divNode.style.fontSize=Font_Size[1].MEDIUM+"px";
        }
        $("a.#S").click(function(){
            var divNode = document.getElementById("text");
            divNode.style.fontSize=Font_Size[2].SMALL+"px";
        }
        var Font_Size= [
            {GREAT:18},
            {MEDIUM:16},
            {SMALL:12}
        ];
    }
</script>
它放在div后面

<script type="text/javascript">
        $(document).ready(function(){
            $("#G").click(function(){
                var divNode = document.getElementById("text");
                divNode.style.fontSize=Font_Size[0].GREAT+"px";
            })
            $("#M").click(function(){
                var divNode = document.getElementById("text");
                divNode.style.fontSize=Font_Size[1].MEDIUM+"px";
            })
            $("#S").click(function(){
                var divNode = document.getElementById("text");
                divNode.style.fontSize=Font_Size[2].SMALL+"px";
            })
            var Font_Size= [
                {GREAT:18},
                {MEDIUM:16},
                {SMALL:12}
            ];
        })
    </script>
加上括号就行

缺少括号