By modern standards the bombs that devastated HIROSHIMA AND NAGASAKI would be tactical- 10- 20 kilotons I think.
i do not believe that an aircraft existed in 1939 which would have been able to carry and drop such a bomb, assuming zero opposition from the Lufthwaffe.
I question whether there would be much 'tactical' advanage from such a weapon, the only way to let it off would have been on Polish territory.
If you are imagining Poland having something like an nuke and a V2 this is truly asb.