These methods applied globally, so unless you wanted to see many sites that doesn't define their font themselves using the same font/font size, don't. You can find a lot of font-changing extensions tho, and most of them had per-site settings. Mind you, that changing the fonts specific to a site can break things, like webfont symbols. If you just wanted to increase the font size, just either use the zoom (Ctrl+MouseScroll) or use another extensions that changes font size without resizing other elements.
Do you annoyed by the site's default font size or the font type? If it's the font size, zoom it. If it's the font type, use this add-on (you need to add manually for any custom font face).