The sweet spot is between 21-35 liters. Within that range, you have enough capacity to pack the essentials without creating an extra heavy pack. That isn't to say there aren't daypacks on either side of that range, though. Running vests are often less than 21 liters; conversely, many bigger packs push 50-55 liters but are still called daypacks. A few larger packs have a sleeping bag compartment, but most packs do not.