Tried a couple different brands, settled on the Topeak simply because it mounts on a quick-release. Only takes a few seconds to detach it when I park my bike outside of the office/cafe/grocery store. The largest bag I've been able to find locally is a medium. Even with it not being a large, I carry the following items with me at all times:

- Multitool
- Spare tube
- Tire levers
- 3 AAA batteries
- Patch kit
- CO2 pump + 1 canister

And I still have room for an energy bar. The specific bag I have is the medium expandable aero-wedge, link here: http://www.rei.com/product/710406