Change font size of all elements inside a div

You can do this with css:

#menu {
   font-size: XXX;
}

Yes you can use JavaScript and or jQuery to do what you want, but why wouldn't you just use CSS like suggested?

or you can try this:

<Style>
    /*Assumed everything is inheriting font-size*/
    #menu{
      font:12px;
    }
    /* Force all children to have a specified font-size */
    #menu *{
      font:14px;
    }
</style>

<script>
    //JavaScript
    document.getElementById('menu').style.fontSize = "14px";
    //jQuery
    $("#menu").css({'font-size':'14px'});

</script>

jQuery Example

$('#menu').nextAll().css('font', '14px');