SO jQuery Fiddle
Test Fiddle mainly for SO Questions jQuery 1.4.3, UI 1.8.5
HTML
<link rel="stylesheet" href="http://ajax.googleapis.com/ajax/libs/jqueryui/1.8.5/themes/ui-lightness/jquery-ui.css">
<div id="dropdown">
<button> <span>opa</span>
<ul>
<li>29</li>
<li>16</li>
<li>16</li>
<li>16</li>
</ul>
</button>
</div>
JavaScript
//var count = $("#selected>ul>li").length;
//alert(count);
var count2 = $("ul").children().length;
alert(count2);