Finder always opens as a small window on the top left corner of the screen
It may be that someone or something has changed the permissions on the .DS_Store file in that particular directory.  Changes you make to window geometry and such are stored in that file.  If you made changes while logged in as another account, then that account may be the owner of that file.
But if you type open /tmp in Terminal, and you get the same window geometry, it’s probably something different.