Page formatting messes up when font size increases

I have recently added a plugin called WP-Accessibility to my wordpress website, which amongst other features has the ability to increase font size at the user’s request, however, when font size is increased the page formatting messes up due to the text. For example, my horizontal nav bar expands to 2 lines to accommodate the links. Is there a way to keep the formatting intact when the user increases the font size? How do I go about it?

Thanks