Screen suddenly too big for external monitor

I have been using my MacBook Air (circa 2011) with an AOC monitor attached for at least 3 years now. Have never had a problem with it.

Suddenly, this week, it seems that the home screen is about 10% too big for the monitor. In other words the top and sides of the home screen don't show up in the monitor and so only the top of the dock is visible at the bottom of the screen and the Apple Icon menu on the left is totally off screen.

I've checked display resolution-- nothing different there. Restarted the computer. Reset PRAM. Unplugged the monitor and removed and reattached at VGA cables.

There's nothing in the settings of the monitor that allows me to shrink screen size-- I can only move it to the left or right or up and down a tad.

Any thoughts on what might be causing this? I have not uploaded any new software or made any other changes that I can think of. I am running the latest version of El Capitan.

Thanks