Is there an easy way to make Sub menu/Drop down menu full width?

Hi I have been looking through other posts but cant seem to find what I am looking for, I was hopeing you could help.

Is there some css code that will make the drop down / sub menus full width?

I am looking at creating a menu similar to this one http://www.blackpoolzoo.org.uk/

any help with this would be fantastic :)