Rob Bensinger wrote a pretty good summary of consciousness for the LW wiki/tag page. The short version is: When the quarks are arranged in a way that implements an algorithm, and that algorithm has certain properties (though the exact details of those properties are somewhat difficult to pin down, since we don't understand intelligence all that well yet).
I imagine there would be some sort of thermal limit, as above and below certain temperatures the matter required to host the consciousness could not survive. So far we don't have any proof of consciousness existing in matter other than carbon based life forms, and I think - at least for the time being - it's fair to exclude all quantum particles outside of the earths atmosphere from being considered sentient. Obviously the astronauts on the ISS would count as sentient as well, but the think a good line for starters to draw is the boundaries of the Earth's gravity. Quantum entanglement might also increase the amount of consciousness in the universe though.