The size of your bag depends on your trip and how much equipment you plan to pack. Many people end up packing one large bag and a smaller, secondary bag for other necessities. For example, using both the 20L and 10L Pathfinder Dry Bags are excellent choices for a several-day trip.
If you plan to use multifunctional items and wash your clothes every couple of days, then you could get away with a smaller 10L to 20L pack or bag. If you want to pack a little heavier, then be sure to upgrade to at least a 30L backpack, which can fit your tarp, clothing, blankets, water, emergency kits, headlamps, and other vital equipment.