Usually the best way to enlarge a web page is the zoom feature. This makes the layout larger along with the text, so it avoids unexpected line breaks, content falling off the page, and so forth, which can occur when you just change the size of the text.

Could you try that and see whether it helps? See: Font size and zoom - increase the size of web pages.

On second reading, it sounds as though the problem is with a particular website. If zoom doesn't help, what site is it?

You can use an extension to set a default font size and page zoom on web pages.

• Default FullZoom Level: https://addons.mozilla.org/firefox/addon/default-fullzoom-level/
• NoSquint: https://addons.mozilla.org/firefox/addon/nosquint/

It is better not to increase the minimum font size, but to use an extension to set the default page zoom to prevent issues with text not being displayed properly.