What size bag is suitable for long-distance travel?

If you only use your backpack for more than half a year, you are really nervous. Moreover, it depends on the way, destination and season of your poor travel. It's almost 50L in summer in the north. If it rains in the south, you must change clothes more. . In winter, the north needs to bring more warm things. .

In short, it still depends on the region, season, weather conditions of the day, travel and accommodation methods and other considerations.