As Greg said, it depends on which browser you're using, and might depend on whether you're using Windows, Apple, or 'Nix.

The Ctl+ key makes the whole page larger. Ctl- makes the whole page smaller.

That might be the actual problem, but what was asked was how to make the font (letter size) larger without changing the page zoom.

If, as indicated, it's only in the browser, the setting for font size is probably in a "tools" or "appearance" setting, but different browsers put it in different places. A "zoom" setting usually will appear on a toolbar somewhere so you can single-click it if the problem is really a tiny page rather than just tiny letters.

John