Why is everything bigger on the monitor now?

Hello, everybody

My monitor is now showing everything way too big. This happened right
after I pressed some wrong buttons on the monitor to turn it on while I was
re-connecting it to the computer.

The type, the graphics, everything is much larger. And much less of it
shows without scrolling the picture.

How can I fix this? What settings are involved? Is it in the monitor or
the computer? I have looked at the monitor's owner's manual and can't
figure it out.

I am using Windows XP Professional. The monitor is a Hanns G 19 inch wide
screen model.

Robert