Are you sure you need that many menu items? Even if you increase the width, your users with smaller screen sizes will still see the menu items wrap around (unless you plan on changing the media query widths in the CSS files).
In any event, you can change the default width of 960px to be an 100% width. From your Dashboard, go to Appearances > Montezuma Options > CSS Settings > Choose CSS Grid. Pick one of the 100% grids, either Responsive or not (I would pick Responsive if you’re designing for readability on mobile devices). Make sure you read the paragraphs above and below the radio buttons so you understand what the options mean. Of course, if you pick an option, and it turns out you don’t like it, pick another, play around as much as you want. But if you want the width to span the entire width of the browser window, then pick one of the 100% widths and not one of the 960px widths.
If you have other questions about the Montezuma theme, you can post them on the Bytes for All support forum.