The definition of "small camera bag" is kept vague on purpose, to leave the airline some wiggle room when applying the rule. I would say a safe assumption would be considering a "small camera bag" any bag small enough not to be considered an extra piece of carry-on luggage.
I have successfully travelled with the camera bag pictured below:
TT speed d http://www.thinktankphoto.com/product-images/large/Speed-Demon-V20-1.jpg
whose technical specifications are:
Interior Dimensions: 9.3” x 6.8” x 5” (23.5 x 17.15 x 12.7 cm)
Exterior Dimensions: 12” x 8.5” x 8” (30.5 x 21.6 x 20.3 cm)
Weight: 1.6–2.1 lbs (0.7–1.0 kg)
I have never had any problems.