In general, sports backpacks should be able to hold 25 to 40 liters. That said, finding the best sports bookbag depends greatly on the types of activities you’ll be doing.
For activities that require a lot of gear, you’ll want lots of extra space. In this case, you should be closer to that 40-liter mark. A smaller sports bookbag should suffice for the gym or other activities that don’t require your own sports equipment.