The diameter of Mercury measures about 4880 km. Can you imagine how much that is? I could not. But with www.sciencenetlinks.com you can compare it with the earth:
By the way, if you are interested in the universe, you should go to a planetarium. You can search the next one with maps.google.com. In Augsburg (Germany) it costs less than going to a 3D-cinema, but the movie showen in the planetarium is in 3D.