University of Arizona
The Biology Project: Studying Cells Tutorial: Size and Biology
Get an overview of light microscopes, scanning electron microscopes and transmission electron microscopes. There is also a brief description of sizes for several microscopic objects.
Other
Scimat: Foods Under the Microscope
A technical site that describes different types of microscopes and delves into the chemical makeup of milk, yogurt, various various types of microorganisms. Impressive pictures supplement the text. Links to images of microorganisms.