I assume that you have a scroll mouse (mouse with a wheel)
Hold down the Ctrl key, and then roll the wheel. The font sizes will vary as you roll the wheel, and you should be able to get back to the original setting that you like.
I think when it first happened to me, I might have inadvertantly hit the control key and rolled the wheel without knowing it.
It shocked the hell out of me until Drew pointed out the solution.
DenverDon

