We are big fans of Osprey gear at Unsponsored HQ. So when a new bag was required for mountain bike duties the Osprey website was consulted.
15 litres or there about seems to be the right kind of size for the bulk of riding that we do. Plenty of room for the essentials but not too much room that you end up filling because you can. The Osprey Zealot appeared to fit the bill nicely.