If it's what I think it is, I don't understand why it would affect only this site.

What happens when you click on

View > Zoom > Zoom In

a few times? That's where the controls for text size are in Firefox, anyway. If you're using a different browser you should have something similar but it may not be in exactly the same place.

On the computer I'm using, pressing Ctrl and rotating the scroll wheel on the mouse also changes text size. Once again, this may not work with all browsers (and not everyone even has a scroll-wheel mouse).