The IC estimates that the yield of the Poseidon weapon is 100 Megatons.

To put it in perspective, the largest nuclear weapon ever tested was the Soviet Tsar Bomba at 50 MT. The DGZ was a point on Novaya Zemlya island, about 1200 NM north of Moscow. The shock wave broke windows in villages are far as 430 NM from DGZ. This pressure wave was detected as far away as New Zealand, which recorded it twice as the wave traveled around the world.

The largest US weapon ever developed was a 19 megaton weapon. The reason that high yield weapons were abandoned in favor of smaller yeild weapons is that they are not militarily useful. Poseidon has no military purpose. It is a terror weapon. In fact by it's designed, Russia cannot control the effects that the weapon would cause. It is a terror weapon, intended to punish the world if they cross Russia.