You probaby hit the control button while using the scroll wheel on your mouse (which increases the font size.)
Hold control and scroll back with the scroll wheel to decrease it. I know in Fire Fox hitting CTRL-0 (thats a zero) will reset it to the normal size again.
You can also use CTRL+ or CTRL- to increase and decrease the font
"I am no one special, just a common man with common thoughts. I've led a common life. There are no monuments dedicated to me, and my name will soon be forgotten. But in one respect, I've succeeded as gloriously as anyone who ever lived. I've loved another with all my heart and soul, and for me, that has always been enough." from The Notebook