Strange change to desktop size

Sounds like maybe the display is slightly Zoomed in.

Try holding the Control key and Scrolling in and out. For that feature go to Apple > System Preference > Accessibility > Display.

If that doesn’t help, then go to your mouse or trackpad preference and check the Smart Zoom setting.

Hello, unkleE.

That is an unusual problem to have. It very well could be a monitor issue... or not.

Have you rebooted the Mac to see if the problem goes away? Give that a try.

And restart the monitor, too.

If rebooting your Mac doesn’t fix the problem, see if the problem still happens in Safe mode (hold the shift key down at startup). It takes longer to safe boot so be patient.

How to use safe mode on your Mac - Apple Support 

Safe mode forces the OS to clear certain caches and logs and do other miscellaneous housekeeping. It's non-destructive to user data so is safe to do.  Very often certain weird software behavior is corrected by simply logging in this way as a troubleshooting method. When you're in safe boot, the machine will not be at its best performance, especially with graphics, but that's normal.

Does the problem persist while in Safe mode? 

Exit safe mode by restarting your Mac normally and re-evaluate the issue again after a normal reboot.

I don’t use/have any newer Apple Keyboards, Mice or Track pad to play with the Smart Zoom features, but the Accessibility Control Zoom feature has been around for a long long time.

That's a neat trick! 🙂

You’re welcome.