The text sizes that you see are handled by your browser, so the exact results you get are going to depend partly on what browser you're using. I've been noticing with recent versions of Firefox that if I change the text size for one Psych Central page (for example), all Psych Central pages are affected but not pages at other sites. I seem to remember older versions of Firefox doing it differently.

One shortcut for changing text size is to hold down the Ctrl key and press + or -. Another is to hold down Ctrl and turn the scroll wheel on your mouse. You might also want to compare how different browsers apply your settings and perhaps choose the browser that does it the way you like best.