The size of microscopic objects is specified in microns - a micron equals 1 millionth of a meter, or 1 thousandth of a millimeter. Most of the protozoa and microbes shown here are much smaller than 500 microns, and are therefore considered truly microscopic, and can be viewed only with the aid of a microscope.
Some Examples Let's begin with a really small example.
A human sperm is 2.5 microns across its body - clearly invisible and requiring at least 400x magnification to see its shape and movement by flagellation clearly. The Rotifer shown here, however, is probably smaller than 200 microns. The fascinating Vorticella never exceeds 150 microns, so it requires a microcope for viewing. A 100x magnification would be sufficient to see its cilia swirling water and food particles into its huge mouth. The interesting Actinosphaerium grows between 200 and 800 microns, the larger being visible with the naked eye.