If you're seeking a carry-on backpack, you should keep it under 40 liters to ensure it will work on all airlines. To ensure it will fit under the seat or in the overhead compartment, it also should measure less than 22 x 14 x 9 inches. If you are willing to check your backpack, you can carry a larger backpack. If you plan to do extended hiking, you probably will seek a 50L or 60L pack.