How do I change it back to a reasonable size?
If you have a mouse wheel, hold down CNTRL with your other hand and scroll the mouse wheel. That will change displayed font sizes up or down.
I didn’t have that problem, but tried it! That is simple and too cool! Thanks.
Yup, thats it. Thank you.
Also in the View menu you’ll find Text Size (in Internet Explorer) or Zoom (in Firefox.) Those settings can help you get back to the default.
You can also try CNTRL/- and CNTRL/+ (on the numeric key pad). It works on Firefox, anyway.
It is also worth noting that CNTRL 0 (zero) will return the font to the default size (which may or may not be the size you like it). This does not have to be done with the numeric keypad. Neither do the CNTRL+ to enlarge or the CNTRL- to diminish: the top row keys work fine (and you do not need to use the SHIFT key with the +). Indeed, CNTRL 0 will not work on the numeric pad unless NumLock is on.
Man, this place is really amazing. In four minutes somebody gave the poster a solution, and a few minutes after that, reports back problem solved. And now you’ve got all of these other solutions to pick from as well. Where else can one get that kind of quality service? Price was reasonable too. 
I have this on every computer I use. Being able to set custom zoom levels on a site-by-site basis is a necessity with all the people who design for larger screens.