Mercury is the smallest planet known, with a diameter of just under 4900km. Until it was disqualified as a planet in 2006, though, Pluto held the title at somewhere over 2300km – less than half the size! Thanks to the formal definition of a planet, Mercury is not likely to lose its place at the bottom of the size ranks for a while: we can see quite clearly that there is nothing smaller in our Solar System which meets all three criteria, and it will be a very long time before we can see such small worlds around other stars.
Comments? Questions? Why not mail me at firstname.lastname@example.org