Mercury is the smallest planet. Mercury’s diameter is 4879 km, while Earth’s is 12,756 km. In fact, Mercury is not much larger than our Moon, which has a diameter of 3475 km. It is even smaller than a couple of other moons in the Solar System, such as Ganymede (of Jupiter) and Titan (of Saturn).
How do the planets size up? Check out the "Planet Size Comparison" animation. Click on the image above.