I'd say broad shoulders is more subjective than just a number. It depends on your height, waist etc. Shoulders are broad proportionally to ones body. So a 6'5" guy with a gut would need huge shoulders, while shorter guy with low body fat might seem to have broad shoulders even if they are much narrower than the 6'5" guy with a gut.
I'd like to hear how others look at this, as I know this is just one opinion.