The screen resolution is there so it all depends on screen distance as well as your eyes.
I connected mine to test the feature out and the size didn’t bother me for the duration I tested. (The horrible stream quality did bother me though, but that’s because I’m testing on an unsupported mac)
I have a Mini 5 and a hi-end 13" HP Windows Notebook/Convertible. After I bought the Mini 5, I installed the Duet app on both devices, which allows the Mini 5 to act as a secondary display for a Windows PC or a Mac. The program worked fine, and indeed the Mini served as a mouse-enabled secondary screen, using a standard lightning to USB-A cable. However, I will only use that capability when absolutely needed when on the road for a very good reason...........the small display of the Mini coupled with the screen resolution of PC, gave me eye strain and a headache both times I used the Mini in that fashion. This could also be the case with the Mini as a secondary display for any PC, including a Mac. Maybe sidecar will not have those issues, but beware.