The accordion has no resizable feature, but you can place it in a resizable container to get this feature.
<div class="easyui-resizable" data-options="onStopResize:function(){$('#aa').accordion('resize')}" style="border:1px solid red;width:500px;height:300px;padding:10px">
<div id="aa" class="easyui-accordion" fit="true">
<div title="About">
</div>
<div title="Help">
</div>
</div>
</div>