You can use accordion for designing this menu. With contents in container area it can be made 100% width.
Examples :
http://widgets-musethemes.businesscatalyst.com/tb019.html#sthash.FhGbFxxT.dpbs
http://www.musegrid.com/adobe-muse-widgets/mega-menus-one
http://muse.adobe.com/exchange
Thanks,
Sanjit