The Molecular Expressions Web site at Florida State University has a really cool interactive java tutorial to help get across the concept of orders of magnitude in relation to the relative size of objects in our world and universe:
View the Milky Way at 10 million light years from the Earth. Then move through space towards the Earth in successive orders of magnitude until you reach a tall oak tree just outside the buildings of the National High Magnetic Field Laboratory in Tallahassee, Florida. After that, begin to move from the actual size of a leaf into a microscopic world that reveals leaf cell walls, the cell nucleus, chromatin, DNA and finally, into the subatomic universe of electrons and protons.
In addition, this site has a plethora of information on microscopy, including a microscopy primer, a miscroscopy museum, a photo gallery and even a list of web resources. You an even try virtual microscopy with interactive java tutorials. For example, you can use scanning electron microscopy to get an up close look at a jellyfish or a gecko’s foot under the microscope.