When display the menu item, call 'resize' method to resize the drop-down panel.
<a href="#" class="easyui-menubutton" data-options="menu:'#mm'">About</a>
<div id="mm" class="menu-content" style="width:400px;" data-options="
onShow: function(){
$(this).find('.easyui-panel').panel('resize');
}
">
<div class="easyui-panel" title="Register" style="width:100%;padding:30px 60px">
<div style="margin-bottom:20px">
<div>Email:</div>
<input class="easyui-textbox" data-options="prompt:'Enter a email address...',validType:'email'" style="width:100%;height:32px">
</div>
<div style="margin-bottom:20px">
<div>First Name:</div>
<input class="easyui-textbox" style="width:100%;height:32px">
</div>
<div style="margin-bottom:20px">
<div>Last Name:</div>
<input class="easyui-textbox" style="width:100%;height:32px">
</div>
<div style="margin-bottom:20px">
<div>Company:</div>
<input class="easyui-textbox" style="width:100%;height:32px">
</div>
<div>
<a href="#" class="easyui-linkbutton" iconCls="icon-ok" style="width:100%;height:32px">Register</a>
</div>
</div>
</div>