Calculating measurements under a microscope
You have an object under the microscope. The field of view is a circle of light.
Using a low power objective, place a steel rule under the microscope. Let's say the field is 2.5mm, or 2500 microns under a 4x objective. Using a 40x objective, therefore reduces the field by 10, so it is now 250 microns
The object is smaller than that. You have already roughly measured the field and for that lens on your microscope
You already know the object's length is smaller than 250Ám. If your calculation gives something bigger than that, there is an error.
Next, to calculate the object's length, first identify what you mean by the length.
Once you have identified the length, figure out what portion of the field it covers. It may help you to imagine dividing the field into half, then dividing a half into two quarters, then dividing a quarter into two eighths, until you get to the size of the organism. That fraction times the field is the approximate length of the organism. Here, the organism's length is about 1/4 of a field, or 0.25 X 250Ám = 62.5Ám.
To get the width: either do all of that again for the width, or eyeball the animal and estimate the width as a fraction of the length. For our organism we might say width = about 1/3 of length.