title_body: Huge icons after update
upvoted_answer: With "automatic scaling", this setting is indeed meant. It would have been more clear if that post had used the actual names, otherwise you can search a long time through the various tabs:  
  
  
    Preferences > Look &amp; feel > Main interface (first tab) > 
                  Adjust for high resolution screens >
           set to off.
  
  
  That you cannot change the value is most likely because you extended Calibre to a second screen to see the buttons (I guess that is the case as your image has a width > 2K). You probably also have font problems when moving the whole preference window to that screen.
  
  To get a functional button, there leave Calibre on the first screen, use the scroll bar at the bottom until you can see "Automatic", and change it to "Off". Calibre will prompt to restart and come back with normal sized icons.