USGS calls it a M_w 4.2.

This curiously showed up about three hours after the test supposedly happened (9:36 PM eastern).

VERY SKETCHY ESTIMATE OF YIELD

According to Wiki's page on the Moment magnitude scale, the relative yield of a nuclear weapon can be determined based on the moment magnitude using the following equation (be sure to read the caveats on the page, this arithmatic is FAR from set in stone):



4.2 = (2/3)(log₁₀(m_TNT/kg))
6.3 = (log₁₀(m_TNT/kg))
1,995,262kg = m_TNT

So it looks to be roughly 2.0kt

Please check this site for a comparison or various nuclear weapons. This one (if I am doing my math correctly, and if the caveats to the equation hold true) does not appear to be that big.