I have no idea what I did. What can I do to restore to normal size? It is happening only on this site.
Hold down Ctrl and hit 0 (zero). That will return your font to default size.
In the future, if you hold down Ctrl and + or - it will change font size up or down. Same as holding down Ctrl and scrolling up or down with the mouse wheel. It's site specific, and nice when you're on a little laptop screen on some sites with tiny fonts.
If you're on a Mac you want to hold down Option instead of Ctrl, but the rest is the same.