Trolley box in the end to buy just how much is appropriate

Common daily use trolley case size is roughly 16 inches, 18 inches, 20 inches, 22 inches, 24 inches, 28 inches, 29 inches, 30 inches. Many of my friends can not help but ask according to different uses, PC trolley case in the end how much to buy?

16-inch PC Trolley Case: Can bring the aircraft, 16-inch international boarding box size, suitable for one-person travel 2-3 days;
18 inch PC Trolley Case: Can take the plane, suitable for one-person travel 3-5 days;
20-inch PC Trolley Case: Can take the plane, suitable for one trip 3-7 days, two travel 2-4 days;
22 inch PC Trolley Case: Do not take the plane, to be checked, suitable for more than 7 days trip, two travel 3-6 days;
24-inch PC Trolley Case: Do not take the plane, to be checked, suitable for two trips for more than 7 days;
28-30 inch PC Trolley Case: Do not take the plane, belong to the shipping box, suitable for overseas shopping, or for a long time travel.