be aware that IE6 and 7 do not work well with menus that use auto or no width.
The interface does not have an option for setting the width of each individual menu item, to give each individual menu item a width, you would need to add inline css:
<li class="parent"><a href="#" style="width:200px;">This is a longer Item</a></li>
however the inline CSS will be overwritten if you edit the menu.