Field of View or Field Diameter is very important in microscopy as it is a more meaningful number than "magnification". Field diameter is simply the number of millimeters or micrometers you will see in your whole field of view when looking into the eyepiece lens. It is just as if you put a ruler under the microscope and counted the number of lines.

The chart below will tell you (approximately) what to expect when looking through a microscope with varying combinations of eyepieces and objective lenses. As an example (in green below), a dual power stereo microscope with 10x eyepiece lenses and 1x and 3x combinations of objective lenses, would have total powers of 10x and 30x and your field of view would be 20mm and 6.7mm respectively.

This means that an object 20mm (2cm, or about 3/4 inch) wide would fill up the whole viewing area at 10x and an object about 6.7mm wide would fill up the whole area at 30x. As you can see, having the highest power may not be best for your particular application. When you move to greater magnifications, you sacrifice field of view.

Other considerations: The working distance is the distance from the bottom of the microscope (lens) to the part of the specimen that is in focus. As you increase the magnification, you decrease the working distance. If you need to work under the microscope, you will need a large working distance. Some special microscopes have extended working distances for these purposes. Zoom microscopes have a fixed working distance throughout the zoom range. When using a 100x objective lens (1000x total power) your working distance might only be 0.04mm (40µm). The lens will be extremely close to the specimen! The working distance and the amount of vertical motion of the microscope will also affect the maximum specimen height. Maximum specimen height is how tall an object you can put on the stage and still be able to focus on the top part of the specimen.

Note: The numbers below will not be the same for all brands of microscopes.

High power microscopes usually have 10x eyepieces lenses, so there is only one column above the 4x objective. If you wanted to look at an amoeba that is 150µm long (0.15mm) then you would use either the 40x or 100x objectives. There are 25mm in an inch and there are 1,000um (micrometers) in a single millimeter.

This page with pennies at different magnifications might be helpful to get a visual idea of what you can see at different magnifications.

Answer to question above: 7.2mm.