are you talking about the mobile menu when the nav is hidden by a "hamburger" icon?
yes. When set to 'Over Right' it is fixed (or appears to be) at 200px. I would like to alter the width.
I have noticed an issue with this as well. You can simply click on the div/container that holds all of your links and create a class for it. From here, you are able to customize just about anything you want. However, I have noticed that the changes I make to it are not visible in preview mode.
For example, I wanted to make the width of the nav bar (open right) 50% the width of the page upon clicking the hamburger icon. It seems to reflect the change until entering preview mode and then it reverts back to the default width.
I was using the navbar widget as opposed to creating it from scratch, so this is more than likely the issue.
I am having this issue as well. The Nav Menu is too large (200px) for mobile phones. Giving "Nav Menu" a class and setting the width or max-width does nothing. I couldn't find anywhere in the forum that this issue is addressed -- curious why hasn't this been answered in almost 2 years?