Backpacks come in different sizes (from Extra Small to Large) and you also find them with different liter capacity (from 30 to +70).
3 days: 30 – 50 Liters
5 days: 50 – 80 Liters
+7 days: 80 Liters and More.
Knowing this, choose one that fits the number of days, the amount of luggage and the logistics that you have completed for your trip.