The smallest snake is proberly the Darwin blind snake Ramphotyphlops tovelli. It is known from only 2 specimens (Is it the rarest as well?), the largest of which was only 12 cm.
Incidently, there is another blind snake Ramphotyphlops braminus that is being spread by human activity and has been introduced into Australia. It has a common name of the flowerpot blind snake as it likes flowerpots or regularly watered gardens. And of the 100 or so animals examined all have been female!