What is the smallest nuclear bomb?
The physically smallest I've ever seen was either a 6 inch artillery shell or the warheads of the Peacemaker missile, both had yields in the 10KTon to 400KTon range.
The lowest yield I've ever seen was the 12 inch Davy Crockett warhead which could be selected for a minimum yield of 10Tons (yes only 20,000 pounds!). There have been bigger conventional explosions.
Nuclear weapons currently produced are between the sizes of a microwave oven and a pickup truck. The smallest have an explosive yield in the sub-kiloton range, and the largest are equal to between 5 and 20 megatons of TNT. For a nuclear bomb, most of the size and weight is shielding, to protect the handlers from the radiation from the bomb core.
Depends "little boy", the bomb dropped on Hiroshima was 3m long with a diameter of 70cm. "tsar bomba", the most powerful bomb ever created is 8m long and 2m in diameter. The "davy crockett", one of the smallest nuclear weapons is only 80cm long and 30cm in diameter. Nuclear missiles may be about 15m long but the warheads are far smaller.
The term atom bomb (or atomic bomb) usually refers to a bomb that obtains its energy solely through the process of nuclear fission. However technically the term is considered interchangeable with nuclear bomb, and can refer to any bomb obtaining its energy through either nuclear fission, nuclear fusion, or any combination of the two processes.