Secondary navigation layout options can be found in Customize > Layout > Secondary Navigation.
The Navigation Width option is the outer container of your navigation. This outer container is what your navigation background color or image is applied to.
Setting this to Full will make your background color/image span the entire width of your screen.
Setting it to Contained will contain it to the width of your container, set in Customize > Layout > Container.
Inner Navigation Width
The Inner Navigation Width option refers to the inner container which holds your menu items.
Setting this to Full will make your menu items span the entire width of your screen.
Setting it to Contained will contain your menu items to the width of your container.
You can choose to align your menu items to the left, center or right.