The French say the North Korean nuke was about one kiloton. The South Koreans say it was half that amount. Apparently the Australians agree with the French. The Russians, on the other hand, say it was fifteen. There are seismic stations all over the planet that were built to measure these things, but the data they collect isn't publicly released. Eventually our own scientists, and the Brits too, certainly, will study the data and decide for themselves how large the yield was.
The difference between one kiloton and fifteen isn't just an indication of the size of the weapon the North Koreans detonated; it's an indication of their success. It's technically more difficult to produce a 1-kt nuke than a 15-kt nuke. Since the North Koreans are new at this, it's unlikely they were trying to build a high-tech low-yield device. If the weapon they detonated produced only 1kt of yield, it was probably because it "fizzled"--the fission reaction stopped. Not that 1kt of yield wouldn't do a hell of a lot of damage; it's equivalent to a thousand tons of TNT, after all. But still, it would mean that the North Koreans detonated a crude device that resulted in partial yield: nuclear nOobs, for now, anyway.
