How to restrict OS X video output to a specific part of the screen
What you're looking for is called adjusting the overscan; this doesn't technically exist on an LCD screen.

However, I think you can probably manually adjust the usable area of your screen using SwitchResX.
If you're running El Capitan, you have to do an additional step to enable custom resolutions called disabling System Integrity Protection (SIP); search their website for a detailed how to.

SwitchResX has a demo version you can test out before buying.