To measure choose the tool,  then click on the image:  

The calculated results will automatically show next to the drawing. Use the formatting tools from Home to make font/line adjustments as desired.

To repeat a measurement type without mouse clicking on the measurement again, press the space bar.

The following measurements can be extended to cover more than a single field of view:

Ruler, Ellipse, Protractor, Circle, Circle center distance, Point to line distance, Parallel Lines.

To extend the measurement, press the middle mouse button on the live image to move the stage in the direction formed by an arrow from the center of the image to clicked point.

Note that extended measurements only works on the live image.