Hamburger Menu Width

Am in the site design process and can’t find how I change the size of the hamburger menu in the top right. This is in desktop mode not mobile mode. It currently fills the entire screen and I want it to slide out/pop out from the right and only take up a percentage of the screen (percentage to be decided).

Please can anyone advise? Alternatively are there any good free plugins for this?

The page I need help with: [log in to see the link]