Click 
.
Position the cursor over the image and click and drag across the area you are measuring.
Release the mouse button. The measurement is displayed on the image. Nodes mark the beginning and end points of the measured area.
Enhancing Images > Using the Tool Box Toolbar > Taking Measurements
Use the Ruler to measure an area in an image.
Calibrate the image before using the ruler.
Drawing and measurements made in the software are done under the responsibility of the user.
To measure an area in an image:
Click .
Position the cursor over the image and click and drag across the area you are measuring.
Release the mouse button. The measurement is displayed on the image. Nodes mark the beginning and end points of the measured area.