Has a diameter of 4,878 km.
The smallest planet in our Solar System, Mercury only has a diameter of 4,878 km. As a comparison, Mercury is not much larger than our Moon, which has a diameter of 3,473 km. One reason why Venus is nicknamed Earth’s twin is because it is almost the same size as our own planet. Mercury is the smallest planet in both mass and diameter. It is about 18 times less massive than Earth. With a diameter of about 3,032 miles (4,879 kilometers), Mercury is not quite two fifths the size of Earth. It is only about a third larger than Earth's Moon. In fact, two moons in the solar systemJupiter's moon Ganymede and Saturn's moon Titanare larger than Mercury. However, Mercury is Mercury is the smallest planet in the solar system. (Pluto used to hold the title, but it was downgraded to a dwarf planet.) Although its surface resembles our moon, the tiny planet has a density that rivals Earth itself. But because Mercury's small mass is enclosed inside of a tiny body, the planet is the second densest in the solar system, weighing in at 5.427 grams per cubic centimeter, or 98 percent of the density of our planet. 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) Radius, diameter and circumference. Mercury's diameter is 3,030 miles (4,878 km), comparable to the size of the continental United States. This makes it about two-fifths the size of Earth. It is smaller than Jupiter's moon Ganymede and Saturn's moon Titan. But because Mercury's small mass is enclosed inside of a tiny body, the planet is the second densest in the solar system, weighing in at 5.427 grams per cubic centimeter, or 98 percent of the density of our planet. Mercury is the smallest and closest to the Sun of the eight planets in the Solar System, with an orbital period of about 88 Earth days. Despite its small size and slow 59-day-long rotation, Mercury has a significant, and apparently global, magnetic field. According to measurements taken by Mariner 10, it is about 1.1% the strength of Earth's. The magnetic-field strength at Mercury's equator is about 300 nT. Mercury is the smallest of the 8 major planets. It is smaller in size than Jupiter's moon Ganymede, or Saturn's moon Titan. But it has a greater mass than those moons due to i … ts higher average density. 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. Mercury has a diameter of 3,032 miles (4,879 km) making it a little more than one third the size of Earth. If Earth were the size of a baseball, Mercury would be about the size of a golf ball. 1  Javascript must be enabled to use Cool Cosmos.  Mercury is the second densest planet after Earth, with a huge metallic core roughly 2,200 to 2,400 miles (3,600 to 3,800 km) wide, or about 75 percent of the planet's diameter. In comparison, Mercury's outer shell is only 300 to 400 miles (500 to 600 km) thick. 
http://www.universetoday.com/33962/diameters-of-the-planets/ http://kids.britannica.com/comptons/article-273928/Mercury http://www.space.com/18647-how-big-is-mercury.html http://www.messenger-education.org/elusive_planet/fastfact_5.php http://www.space.com/18647-how-big-is-mercury.html https://en.wikipedia.org/wiki/Mercury_(planet) http://www.answers.com/Q/What_is_the_size_of_planet_Mercury http://coolcosmos.ipac.caltech.edu/ask/19-How-small-is-Mercury-compared-to-Earth- http://www.space.com/36-mercury-the-suns-closest-planetary-neighbor.html
what size is mercury