There is no definitive answer to this question as the best backpack size for day hiking depends on a number of factors, including the type and duration of the hike, the terrain, the weather, and the size and weight of the individual hiker. However, as a general guideline, most day hikers find that a backpack with a capacity of around 30-40 liters (1,800-2,400 cubic inches) is sufficient for carrying all the necessary gear, food, and water.