A virus is an extremely small, infectious agent that is metabolically inert and only replicates in living hosts. Viruses are so small that you usually need an electron microscope, as opposed to a light microscope you may find in your school lab, to even see one! Basically, the diameters of different viruses fall somewhere in between that of a bacteria on the high end, and that of a ribosome on the low end, which is essentially a range of about 20-300 nanometers in diameter.

To put it into a better perspective for you, an average virus is about eight times bigger than a ribosome, whereas the average bacteria is about 30 times bigger than your average virus. That's like taking a small dog (the ribosome), putting it next to a human (the virus) and putting the human next to a rhinoceros (the bacteria)!

NGSS Link:

HS-LS4 Biological Evolution: Unity and Diversity