Seeing as the bombs that were used on Japan were in the kilotons of TNT and the one that
>>14289552shows is at maximum 1 kilton, even one the size of a watermelon is largely ineffective as a nuclear bomb, and would do more as a dirty bomb than a destructive device.
Einstein thought that nuclear bombs would have to be so large that they would need to be carried by ship, so of course we can make them smaller than what the math shows at first, but anything the size of a coin, as you suggest, would again, be more of a dirty bomb that spreads radioactive material.
