Field diameter is commonly referred to as "field of view," meaning that when you look into a microscope, everything that you see falls within that circular scope of vision. You may want to know the sizes of the objects that fall within the circle, and to calculate that you will have to know the size of the field. To determine the field diameter, the process of calibration of your microscope is imperative for accurate measurements. The following method gives you a good estimate.
Things You'll Need
 Microscope
 Ruler

Make a chart with two columns labeled "Objective" and "Measurement". Label three rows: "4X", "10X," and "40X."

Place your ruler on the microscope stage (the platform upon which objects are laid to be examined) so that the millimeters side is under the viewer. Set the microscope to the 4X.

Look into the microscope. Count the number of millimeter sections that you see at the widest part of the circle. If there is only a part of a millimeter present, round to the nearest half. Record this number in the cell of your chart that is in the 4X row in the Measurement column.

Change the microscope setting (called the "objective") to 10X, which may be the next level up, depending on the make and model of the microscope.

Look again into the microscope. Count the number of millimeter sections that you see at the widest part of the circle. If there is only a part of a millimeter present, round to the nearest half. Record this number in the cell of your chart that is in the 10X row in the Measurement column.

Change the objective to 40X on your microscope. You will not be able to count millimeters at this level of magnification, so instead, divide the number you got for the 4X by 10, since 4 is one tenth of 40. For example, if your number for 4X is 4.5, then 4.5 divided by 10 is .45. Record the answer in the 40X row of the Measurement column.

Place an object that you want to examine at the microscopic level into the slide and clip it onto the stage. Set the objective.

Look into the microscope. Bear in mind that an object under a microscope appears backwards and inverted because of the mirror construction that enables the microscope to magnify.

Estimate how much of the circle the object takes up. Multiply by the number that is on your chart in the objective row in the Measurement column.
Tips & Warnings
 A microscope without any setting magnifies an object 10 times, thus, 4X magnifies 40 times; 10X magnifies 100 times; and 40X magnifies 400 times its normal size.
References
 Photo Credit microscope image by christemo from Fotolia.com