<div class="col-xs-2">
<div class="dropdown" >
<a class="btn-default" role="button" data-toggle="dropdown">
<span><img src="images/nav1.png" class="img2"></span>
</a>
<ul class="dropdown-menu dropdown-menu-right" >
<li ><a href="class.html"></a></li>
<li ><a href="teacher.html"></a></li>
<li ><a href="student.html"></a></li>
<li ><a href="activity.html"></a></li>
</ul>
</div>
</div>
</div>
</div>
dropdown里面ul大小只能调大不能调小么