In the question Will Rosetta have to adjust its orbit around Chury due to the comet's coma and tails? and related answers it is implied that the Rosetta spacecraft will actually orbit 67P/Churyumov–Gerasimenko. When I think of orbits I think of the high speeds and large bodies as a gravity base for the orbit; Sun, planets and such. A comet has a tail because of all of the stuff falling off and falling behind the body of the comet as it orbits the sun. Anything orbiting it seems counter intuitive.
Will Rosetta actually orbit the comet or is that just a figure of speech? Is there some minimum size of body (in our solar system) that can be orbited? Is the smallest size based on the relationship between the two bodies size?