Mega Menu megamenu Yootheme

Layout / Header / Dropdown

Turn on Dropdown full width submenus

But... Untick Align to navbar or all the items will line up on the left of the page.

You can adjust the megamenu to spread larger menus across the page.

 YOOtheme / Menus / 

Set up megamenus by clicking on the menu you want to upgrade and then the builder button beside the item

Width of the dropdown inside the dropbar!!!

Go to Customizer > Menus > [Menu] click on the name of the parent item (not on the "Builder" button), and select Dropdown Stretch: Navbar or Dropdown Stretch: Navbar Container.

Menus / Main Menu / choose your menu item / Nav / Settings / Grid

Changes the number of columns you want for your drop down.