They all require another living cell to reproduce. I would call them the ultimate parasite. They take all they need from the cells they live in and when done they usually destroy the host cell. An example of how small they are can be found at the Virus Ultrastructure site . This site also contains a number of viral images .
Viruses come in a number of shapes and sizes .
To see a virus you need a very special microscope called an electron microscope . Bacteria are the next size up. Go look at Cells alive . It has a great webpage comparing the size of viruses to bacteria. Bacteria like viruses are all around.