system
Given the following post, respond as if you're commenting on Reddit:
human
Before they detonated the first atomic bomb, did they have a good idea of how big the explosion would be?
gpt
The size (yield) of a fission explosion is directly a consequence of how many fission reactions take place, which in the kind of explosive design used in the Trinity test (an implosion design), is directly related to how well the compression of the bomb core is. So they could take various guesses as to how efficient the compression would be, and from that extrapolate to possible explosive yields.

There was considerable uncertainty as to whether the implosion design would work at all, and how efficient it would be. Some of the scientists suspected it would not work at all and there would be essentially no nuclear yield. Bets were taken by various senior scientists as to the possible yield: Oppenheimer (0.3 kt), Teller (45 kt), Kistiakowsky (1.4 kt), Bethe (8 kt), Ramsey (zero), Rabi (18 kt). You can see that most of the ones we know cluster rather low, with the head of the project (Oppenheimer) having one of the lowest (Teller's high bet is perhaps reflective of his own unimpeded enthusiasm for big explosions). 

The test committee that made the final efficiency estimates thought it was likely to be 4-5 kt. 

In the end, it was around 18-20 kt. So several factors higher than the test committee estimate, and higher than almost all of the bets. The bomb worked better than they thought.

One might wonder whether there was danger in being wrong on this, and while being _outrageously_ wrong could have been a problem, the _damage_ of nuclear weapons does not scale linearly but as a cubic root. So you have to multiply the yield by a factor of 8 in order to double the range of the damage. So being off by 4-5X does not really affect the safety at all. They had built in healthy safety margins into their planning with regards to acute effects. (Fallout was trickier than they had a much more ad hoc system for dealing with that.)

The best single overview of the Trinity is the book _The Day the Sun Rose Twice_ by Szasz.