Yes min-width 400px will work as long as that is wider than the minmum width of that nav. You have set the lists to min-width: 8em so you should have set your min-width in em to match the total number of list elements.
In a dropdown its much easier to work with fixed widths and you can avoid issues like the menu dropping.
but how can i put multiple id's in one div?...
You can't do that as ids are unique and there can only be one id per element and unique to that page. An id is a unique reference to that specific element and so cannot have any other ids on it. It would be like putting the number 21 and 72 on your front door - no one would know what's right.