As I wait for the MESSENGER Mercury Orbit Insertion webcast to start, I thought I'd fiddle with some images to point out that Mercury is a bridge between the scales of planets and the scales of moons. Here's a collage of the eight bodies in the solar system that are between 2,500 and 6,000 kilometers in diameter. That's Mercury on the left, then the Moon, then the four Galilean moons of Jupiter, then Titan, then Triton. Both Ganymede and Titan are bigger than Mercury; Ganymede's actually the biggest but Titan appears bigger because of its puffy atmosphere.
All images NASA / JPL / Ted Stryk except the Moon (Alfredo Balreira) and Titan (NASA / JPL / SSI / Emily Lakdawalla). Montage by Emily Lakdawalla.
Some similar-sized objects in the solar system
These are the eight bodies in the solar system that are between 2,500 and 5,300 kilometers in diameter: Mercury (4,879 kilometers), the Moon (3,475), Io (3,639), Europa (3,125), Ganymede (5,265), Callisto (4,819), Titan (5,150, but it appears 5,550 with its atmosphere), and Triton (2,706). The next biggest body is much larger Mars, at 6,794 kilometers, and much smaller Titania, at 1,578.