or use Command+ and Command- for Safari.
(hold down Command key then + to increase, - to decrease magnification)
That will increase the size of everything on the page, not just the font, but so does the button.
Other applications may have a setting. Go to the bold application name next to the apple (upper left) and choose Preferences. Look for a font size setting in there. It won't be global. That is, you'll have to do it for every application.