Question: is a 20l backpack big enough?
Answer: Daypacks. Most daypacks range from 20 liters to 35 liters on the high-end. While a smaller sized backpack is usually sufficient for 1/2 day hikes, you'll want a larger pack in the 35-liter range for all-day hikes so you can carry extra water, food, clothing, and the 10 essentials.