Return my windows to their original smaller size.

I have had a Mac Book now for 2 days. I am accustomed to a PC. This
is VERY new. I did something inadvertently yesterday that has made
each winow that opens TOO big. It has given me a scroll bar at the
bottom that was not previosly there. As a result to do any reading I
must be constantly scrolling.

I have gone through each item that I can find anywhere to make
achange and fix this without success…any one who can help?