This is a bit like the Mega Menu in that you can open a dropdown and find several sublinks arranged horizontally. The menu is put inside an .outer-subnavwrap which makes it easier to limit the width, move it around or make it fixed. If some space above the menu is necessary, it can be set as a margin to that outer wrapper.
Start limiting the width by giving the outer container a max-width of, say 75%, as in this example, margin left and right to auto. The margin top is optional.
Then move to the element .subnav-content. The 75% width component needs to have the remaining 25% split between the left and right side as distance from the edge. You may have to adjust the values a little, like rounding off the decimals e.g. This can be seen under Position on the Styles pane for the element .subnav-content. You can change the width as you like by observing the logic in those settings.
The full width component has been made sticky to the top of the page. That means it will always be visible on the page.