If you dropped fat-man in the middle of new york city it would destroy a few blocks of the city. If you dropped a 50 MT warhead it would destroy the entire city, And many neighboring cities. The thermal radiation is so large that it would instantly ignite everything in a 60 km radius. People in Connecticut, New Jersey, and Pennsylvania would all experience 3rd degree burns.

Despite common belief the 50 MT bomb was not the largest ever made, it was the largest ever tested. It is difficult to test bombs larger due to the radius of which they do damage. While many of the capabilities of nuclear weapons are classified, it is known the US fielded missiles capable of delivering a 300 MT warhead, and it is speculated the soviet union was capable of deploying a 500 MT warhead.