Customize MegaMenu width

Dear GeneratePress,

I’d like to change the width of my megamenu to something similar like on this site: http://www.moneyunder30.com. Their megamenu is not the full width of their site, but there is empty space/padding on both sides. See here: https://imgur.com/Q5Sh3xD

Could you please help how I can customize the width of my website similarly?

I used the Navigation-as-a-header option suggested above by Leo, but the megamenu becomes full-width which I don’t want (I’d like to have spacing on both sides).

See picture: https://imgur.com/xmW0Mcu

If I tick off the Navigation-as-a-head option, then the width of the megamenu is not wide enough and my text does not fit.