Which is the smallest venomous snake?

That is a question that is not so easy to answer. Technically, the smallest venomous snake would probably be the Crowned Black-headed snake (Tantilla planiceps).  It is a small rear-fanged snake found in the southwestern United States. Though not a dangerous snake, it does have a gland called a Duvernoys gland that secretes venom into the mouth where enlarged grooved teeth in the back of the snake's mouth "chew" it into the prey animal. This snake rarely exceeds ten inches in length, and it feeds on small earthworms, centipedes and other small insects.

Now, as far as the smallest dangerous or medically important venomous
snake that would depend on where in the world you look.  In North America
it would be the Sonoran Coral Snake, found in the south-western U.S. and Mexico. The Sawscaled viper of Africa and Asia is also rather small and would be the smallest on those continents.

Pigmy rattlesnake (Four sub-species; Sistrurus miliarius miliarius, Sistrurus miliarius barbouri, Sistrurus miliarius streckeri, Sistrurus ravus ), has a "typical" length very similar to the coral snake. Though, the pigmy rattler is not known for being a 'deadly' species, it is a viper and some serious bites have been documented. 
- Information submitted by Tim -