Resolution when connecting to a headless Mac Mini
I’m not sure how you could do this within macOS, but I know that every portable Mac I’ve owned since the PowerBook G4, as well as my current Mac mini, all run a tool called SwitchResX. It has saved me so many times in connecting to projectors, screens, etc. and in exactly the same situation you’re in where I wanted to have my MacBook Air screen resolution exactly matched by the Mac mini so that I had a full screen experience. 

Go and have a look at SwitchResX over at http://www.madrau.com. There is a fully functional 10-day trial version, and €14 buys you a license. Well-worth the expense in my opinion!