We've found that 20L to 30L is sufficient for most purposes. You may need more than 30L, so you might consider getting a bike messenger. Keep in mind the fact that the bigger your bag, the more weight it is, the greater the load on your hands and soft tissues around your bones. It is possible to pack light and have the ability to access it when you are in need. However, having a large bag may encourage you not to use some of your essentials. A lightweight bag with a low capacity might make it more comfortable to ride on, but you do not want to have to choose between food and clothes. You can always go with the larger option, even if you are unsure. It is better to have enough capacity than not use it.