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.