Q: Why cant I change display on my mac mini. Not sure what I did but display takes up whole screen. How can I put it back to normal size?

press escape key.

Try Control + Command + F keys pressed at same time

That toggles full-screen mode

If that doesn't work try a two-finger double tap. If that works open system preferences for the trackpad and uncheck the box for smart zoom.

Can you post a screen shot of what you are seeing?

The suggestion may imply the availability of an accessory USB Trackpad, or if you

happen to be using a portable that has built-in trackpad, then that is possible...

Otherwise, the exercise only invites an additional cost

of accessory purchase, & provides no immediate cure.

Mac keyboard shortcuts - Apple Support

OS X : Reset your computer’s PRAM

PS: there may be some valid suggestions as of yet untried, in this search:

https://duckduckgo.com/?q=how+to+reset+display+resolution+on+startup+Mac+OS+X&t= ffsb