When you draw a line with the Measure Button turned on, V1 Pro will place a digital measurement along the line. By default, the units of this measurement are “pixels” on the display (so if the V1 Pro's video window is 840 pixels wide and 630 pixels high, a horizontal line drawn from the leftmost point to the rightmost point on the window will measure 840.00).
You may “calibrate” these measurements by holding down or right-clicking the Measure button.
The Calibrate window instructs you to “draw a line of known length” and then set a control to match this value. As soon as you make a change, you will see all on-screen measurements immediately updated.