His final estimate was up to around a kiloton, likely a bit less. Another redditor also tried his hand and came up with an estimate of around 700 tons.

Both of those numbers sound reasonable. It's obviously a huge explosion, but the building right next to the warehouse was still standing, so it couldn't have been on the order of multiple kilotons.

Some people have speculated about the possibility that it was a nuclear weapon. While it is possible to create a nuclear explosion that small, there's no evidence of any radiation at the site, and the Lebanese government has already come out and said that a large supply of confiscated ammonium nitrate was being stored in that warehouse.

For reference, here is a list of the largest conventional explosions ever recorded. They go up to about 4 kilotons, several times the likely range of this one.