The Moon has a mean diameter of 3,474 kilometres. Mercury's mean diameter is 4,879 kilometres (40% bigger). Pluto (which of course is no longer a planet) has a mean diameter of 2,377 kilometres, which makes it 37% smaller; so Pluto is actually closer in size to the Moon than Mercury is.
Two moons of the Solar System are larger than Mercury: Ganymede (of Jupiter, mean diameter 5,268 kilometres) and Titan (of Saturn, 5,149 kilometres). Two more of Jupiter's satellites are larger than Earth's Moon: Callisto (4,821 kilometres) and Io (3,643 kilometres). The smallest of Jupiter's Galilean satellites (Europa, 3,122 kilometres) is also larger than Pluto; and so is Triton, of Neptune (2,707 kilometres).
The Quiz Monkey has a diagram illustrating the relative sizes of the Solar System's major satellites and minor planets, and other bodies of similar size (including Mercury).
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