Apparently, a problem with Windows display scaling (or which specific GFX card, i think, dunno, everything works fine on all 3 of my PCs ?-)
So, now the config window allows you to select WINDOWED instead of FULL-SCREEN mode. Hopefully, adequate until a real fix becomes apparent ... ?-)
https://answers.microsoft.com/en-us/windows/forum/windows_other-gaming/problem-screen-size-is-smaller-while-playing-any/f714a6fa-ee48-48c7-b96d-44ae7e38744e