Ron’s code above should work for most situations assuming that you have position relative only on top-nav and not on the child list items. The drop menu is then the same width as the parent top-nav and assuming top-nav is fluid then the menu will size with the parent.
If you can get it working in your fiddle first then you can try and replicate in your actual code.
If you have a link to the actual site then we can probably give more specific help.
You also have the option of using media queries to change the width of the menu when it starts to stick out of the viewport and then you throw in a smaller width accordingly (e.g. shorten the width by a couple of hundred pixels and adjust internal elements as suitable). This would work with your existing code as you just manipulate the width each time. With two or three media queries you should be able to make it fit across the range.