Luggage and the weight

According to China Southern airlines, the free baggage allowance is usually 40kg for the first class, 30kg for the business class and 20kg for the economy class. While on China-USA flight routes, the free baggage allowance is 2 pieces for each economy class ticket holder, and the total weight should be less than 23kg. Besides, the three sides of each piece of baggage could no more than 158cm.