The dwarf planet Pluto is 2274 km across and it is located between 4.1 and 6.8 light hours from Earth. But we do not have any close-up images of it.. yet. In 2017, the New Horizons space probe, which left Earth in 2006, will fly by Pluto and send photos back home.
In this image, Emily Lakdawalla of the Planetary Society has assembled images of the other small bodies in the solar system that have already been visited by spacecraft. Their relative sizes are to scale. Since this image was made, the Dawn space probe has arrived at the protoplanet (4) Vesta, and by the time you see this, perhaps other worlds will have also been visited for the first time.
The moons of the giant planets, like Jupiter's Europa and Saturn's Titan, are also worlds in their own right. Ganymede and Titan are larger than the planet Mercury, although less massive.