How big should a diaper bag be?

We have a small one-strap back pack that we use. It holds about 6 diapers, wipe container, an outfit, my nursing cover, and random odds and ends such as gas medication, powder, etc. When we are going out with my daughter, I just stick my wallet and phone ( I keep a lip loss in the diaper bag) in the bag and go. We have a bigger bag for extensive trips...but I enjoy having the small one for everyday use.