Start with approximate relationship between blast radius R and blast energy E:
|heat capacity ratio||1.4|
A kiloton of TNT is equivalent to approximately , so yield in kilotons is
Plugging in the numbers from the released pictures, with radius measured from the thin blast wave line rather than the edge of the fireball:
The average over five images is 19 kT, which turns out to be pretty close to the now declassified value.
Source: Sanjoy Mahajan, The Art of Insight in Science and Engineering, chapter 5.2.2
Pictures from Atomic Archive