The classification of backpacks is conditional and largely depends on your personal characteristics and preferences. Traditionally, items with a volume of up to 25 liters are considered suitable for day trips. If you plan to go somewhere with an overnight stay, you should take a slightly larger backpack (35 liters). For two-day hikes, 40-50 liter backpacks are suitable, and for trips of 5 days and more, you should take at least 65 liter backpacks. Of course, there are products with a larger volume (up to 150 liters), but scientific research and my experience show that a big load weight can negatively affect your endurance and health.