The smallest answer possible is two letters long and is "No". There is no other word that is one letter long that can be used as a complete answer to a question, hence the smallest answer (in English at least) is two letters long.
smalest
1 cm
I don't know if it's the smallest, but the Kahr P45 is pretty small.
Vatican
The hammer in the ear
85 millimeters
It is: 1
The soprillo saxophone.
He Pingping, from China
Vatican City
It is Vitican City.
Vatican City