Increase Default Font Size Site-wide

I read two or three other threads where others found the font size to be a problem, too, and I could not for the life of me figure out which solution to use, and which codes went where. Is there now a way to change font sizes for the site, yet? How can I increase the font site-wide? A couple of suggestions in previous discussions was to "just add this css to your template." Which template? Exactly?

Thank you in advance!
troy