Full Width submenus are the same width as the menu bar. By default, all mega submenus are full width (meaning the full width of the menu bar). They will remain that width so long as the Submenu Position is set to “Full Width”.
If you want your submenus to be wider than the menu bar, please see Submenu Bounds (Maximum Submenu Width)
Natural Width submenus are sized to the contents of the submenu. Changing the Submenu Position to anything other than Full Width will create a Natural Width submenu.
Note that if you are using a Natural Width submenu, your submenu columns will need to be a natural width as well. You can’t divide it into grid columns via the Layout > Columns Width setting, as the columns depend on the width of the submenu (and a natural width submenu depends on the width of the columns – so this creates a circular reference that not all browsers handle well).
If you would like to set an explicit width for your submenus, you can do so in the Menu Item Settings > Submenu > Submenu Width
Generally, the only reason you should set an explicit submenu width is if you want to be able to use grid columns inside a submenu that is less than full width.