what size is mercury
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 Earthâs twin is because it is almost the same size as our own planet.s a comparison, Mercury is not much larger than our Moon, which has a diameter of 3,473 km. One reason why Venus is nicknamed Earthâs twin is because it is almost the same size as our own planet.