So how big was that bang?

Perhaps this is more than appropriate for our age: immediately following an explosion, much debate was devoted to the bomb's power.

The New York Times:

“We have assessed that the explosion in North Korea was a sub-kiloton explosion,” said the intelligence official, speaking on condition of anonymity because of the sensitivity of the issue. He added, “We don’t know, in fact, whether it was a nuclear explosion.” He spoke as intelligence analysts in Washington were in the early stages of assessing the explosion.

A one-kiloton blast would be extremely small for a nuclear weapon. But regardless of the size of the blast, the North Korean announcement reverberated throughout the world of diplomacy, and seemed likely to be felt in American domestic politics as well. There were suggestions, moreover, that the Communist state might be preparing a second test.

The American assessment of the blast contrasted with that of Russia, whose defense minister, Sergei Ivanov, told the ITAR-TASS news agency that the Moscow government believed the strength of the weapon was from 5 to 15 kilotons.

Such assessments can be dubious. They involve the analysis of geological conditions and the earthquakes that correspond to the explosion. Moreover, we do not know how this device was positioned.

Here's something on the India/Pakistan blasts from Arms Control.org:

From the technical perspective, it is generally accepted that a first-generation nuclear weapon of simple design and modest yield can be developed without testing. For weapons that are readily deployable on ballistic missiles or on the battlefield, as well as more powerful thermonuclear weapons, testing would almost certainly be necessary to attain confidence in their capability. For effective deterrence of such weapons development, the baseline goal for the seismic monitoring system has been detection and characterization of explosions corresponding to a magnitude of about 4 and larger. This threshold roughly translates to an equivalent yield of 1 kiloton (1,000 tons) of high explosives, if the energy of the explosion is efficiently transmitted, or "coupled well," to the surrounding rock. In many places around the globe, including the locations discussed in this article, the monitoring threshold is significantly better than the IMS baseline.

[...]

Although India's tests reportedly took the U.S. intelligence community by surprise, the seismic waves that they created were recorded by 62 stations used by the prototype IDC. Indian officials could not hope that a seismic signal with magnitude 5.2 would evade detection, and the seismic waveforms were unmistakably explosive in nature. (See Figure 4.) In addition to confirming that a nuclear explosion had occurred at the Indian test site, the seismic data allows independent evaluation of the validity of India's claims.

Most of the energy released in an underground explosion (as is the case with an earthquake) is lost as heat or is expended to fracture rock close to the shot point. Less than 1 percent of the energy goes into seismic waves that travel long distances. The effectiveness with which explosions generate seismic waves strongly depends on how well the explosions are coupled to the surrounding rock, and how efficiently the region transmits seismic waves. Extrapolating from U.S. explosions at the Nevada Test Site and from measuring yields of Russian tests in Kazakhstan and Novaya Zemlya, French tests in Algeria and Chinese tests at Lop Nor, a magnitude 5.2 seismic event at the Indian test site should correspond to an underground nuclear explosion with an equivalent yield of approximately 12 kilotons of high explosives. Without explosions of known size with which to calibrate the relationship between seismic magnitude and explosive yield at this particular site, the 12-kiloton estimate has significant uncertainty. The true yield could be as low as 5 kilotons or as high as 25 kilotons because of the previously described geological factors.