How to change default font size in tinymce?

<script type="text/javascript">
    tinyMCE.init({
        mode : "textareas",
            setup : function(ed)
            {
                // set the editor font size
                ed.onInit.add(function(ed)
                {
                ed.getBody().style.fontSize = '14px';
                });
            },
            });
</script>