Q: Why is my whole computer screen zoomed in?

If you have a roller ball or scroll-wheel mouse, this may affect some applications

should the Control key be pressed while the mouse wheel is turned up or down.

And if you chose to install third party mouse software, that may add a layer of

difficulty since most mouse products generally just work natively in OS X.

In different OS X versions, and hardware vintages, the exhibited effects can differ.

• Mac keyboard shortcuts - Apple Support

And if the zoom setting was also enabled in the Accessibility (universal access)

the mouse may be less manageable until that is disabled; if those settings weren't

a cause of the problem originally.

• Mac keyboard shortcuts for accessibility features - Apple Support

There may be a keyboard shortcut, to disable the zoom effect, depending on what

the initial cause of it was. Several Accessibility shortcuts are in its Preferences...

Some applications may utilize custom keyboard shortcuts for use within them.

Not sure if you could start up the Mac in Safe boot mode and then try to see if

repair disk permissions may help, then restart normally after exiting open apps.

• Try safe mode if your Mac doesn't finish starting up - Apple Support

Or just restart the computer. If you cannot use the keyboard to access menu

options, you could press and hold down the power key for 15 seconds or so.

The zoom settings should not be default, at least not in Safe boot.

Good luck...!

One thing to try is to check out/experiment with the screen resolution via System Preferences, Displays.