I highly, highly recommend this bag:
Tenba - Black Label: Small Photo Satchel
It's very small, but the quality of work is simple the best I've ever seen,
much better than ThinkTank and Domke.
The smallest size is made for a compact system like Leica M
and it fits my Pentax K7 with 4 pancakes like a glove. The problem with the smallest
bags is usually the opening, Think Tank UD 10 s@cks, as it has a tight zipper
and in a small size bag it can scratch the camera very easily.
Tenba has a wide, no zipper opening, but it closes very securely with two large clips.
Check it out!