Strange Font Question

I have an aluminum iMac running leopard. Recently, I tried to modify the font size in Safari to make it easier to read web pages, and created some issues for myself -

I increased the default font size. But then I found that if I printed something on the computer, it came out too large. So I starting fooling around with the font settings and zoom.

Ultimately, I decided to go back to a larger font, 18 - but when I did that, it now looks tiny on the screen, maybe 10, and I have to zoom to enlarge it.

What controls the default font? I think I would like to start from scratch but can't figure out how to go back.

Thanks.

Unfortunately I have not been able to figure out the solution to this problem - and am still noticing disparaties between what I print, see on screen, and (I think) the font size I'm choosing in the preferences. Any suggestions including how to go back to the beginning would be very much appreciated. Thanks!