I was thinking that it might be helpful if you added a couple of extra menu item properties like minitemheight and minitemwidth. That way if your menu items were small you could still have a set width and height, but if you had an item that was larger than that in minitemheight or minitemwidth it would dynamically make it wider/longer.
Finally got around to this problem in the To-Do list.
Anyway, setting itemwidth will fix the width so there's no need for a minwidth property.
As for minheight - This too isn't going to be possible because the menu item needs to expand to allow room for the text. It's always set to it's minimum but can't really be made smaller.