How do I reduce the screen size on my MacBook Air?

Best thing to do is to call 1-800-MY-APPLE which is Apples free tech support over the phone.

Sounds like you may have zoomed in with either the mouse or thru screen settings.

I'm not sure what you're experiencing but have you tried Command + 0 (Zero)? See if that helps.

Hold down the control key and drag down the track pad with two fingers.