Because the individual cells of any organism are too small to be seen with the naked eye, we must use microscopes to magnify them. We can view a cell at a magnification of up to 1000x under a light microscope, but we can’t gauge its actual size just by looking at it. However, we can accurately estimate a cell’s size by doing a little bit of math.
Look at the revolving nosepiece, or turret, of your microscope and identify the objective lenses. Typically, you will find a 4X, 10X, 40X and possibly a 100X objective lens.
Place the 10X objective lens in position, leaving off the microscope slide for the moment.
Turn on the microscope's light source, and adjust it for eye comfort while looking through the eyepiece lens. You should see a white circle of light. This is the “field of view” of your microscope.
Place your metric ruler on the microscope stage, and move it into a position where you can see it clearly. Align one side of the ruler with the left edge of the field of view, and measure the entire field of view. This measurement is typically 1.4mm to 1.5mm. Given that 1mm equals 1,000 microns, 1.4mm equals 1,400 microns.
Place your prepared slide on the microscope's stage, and use the “coarse” and “fine” adjustment knobs to focus on your specimen.
Estimate how many cells laid end to end it would take to equal the diameter of the field of view. Then, divide 1,400 microns by this number to obtain an estimate of the cell's size in microns.
For example, suppose it takes 8 paramecia laid end to end to equal the diameter of the field of view. If you divide 1,400 by 8, you get 175. Thus, the size of a single paramecium is approximately 175 microns.
Improve this measurement by changing to the 40X objective lens. This will give you a field of view that is one-fourth that of the 10X objective lens (10X/40X=1/4). Dividing 1,400 by 4 indicates that the field of view for the 40X lens is 350 microns (1,400/4=350).
Estimate how many cells laid end to end it would take to equal the diameter of the field of view. If 2.5 lengths of the organism would span this diameter, you can divide 350 by 2.5 to get a closer approximation of the size of the cell (i.e., 140 microns).
TL;DR (Too Long; Didn't Read)
You can use an ocular micrometer to measure cell size. An ocular micrometer is basically a tiny ruler etched into one of the ocular lenses; it can give you a better estimate of the size of a cell, provided you calibrate it with a stage micrometer, which is a microscope slide that has a scale etched into its surface.