In bug 31785, icons were defaulted from Large to Automatic to use OS settings to determine whether to use small or large icons. Now 4 years later, Automatic is no different then setting it to large, so in order for the Automatic setting to truly be Automatic, it should adapt according to the user's screen resolution. So I'd like to suggest that if a user's screen resolution is less than 1280 pixels in width, which is approximately 10% of internet users today, the Automatic setting should show them small icons and not large.
This might very well be a dupe - but marking as NEW for now at least.
Kendy removed the code in 'Automatic' that caused small icons to appear on Windows (bug 82309) in the below commit, so i'd assume that adjusting the removed code to check against the screen width would be what is needed, of course also taking into consideration hidpi.
Migrating Whiteboard tags to Keywords: (needsDevEval, topicUI)
*** Bug 67847 has been marked as a duplicate of this bug. ***