Basically the 50 is the biggest you can get away with if the bag is fully stuffed. But a 55 or 60 is fine as long as you can squish it into the tester cage thing, i.e. as long as it's not full.
One advantage of getting a used one is that it's not such a disaster if they do make you check it.
Mine is the 50 which I think is the perfect size for me. It's small enough to get away with using it day to day, but big enough to be useful for hand luggage / nights & weekends away. If I wanted it as a true weekend / travel bag I would get a 55 or 60, but then it would be less versatile imo.