Rami Arieli: "The Laser Adventure" Chapter 1.2.2 page 3
Spot Size Measurement:

R = Radius of the illuminated spot at a distance L from the laser (see figure below).

If the spot size measurement is done near the laser (where the spot is small), then the size of the beam at the output of the laser needs to be taken into account:
 

 
Because the laser radiation has a very small divergence, the small angle approximation can be used. Thus, we have set the tangent of the angle equal to the angle.

On a screen, the laser produce a spot. The diameter of this spot (2R) determines the spot size.

When the measurement is done very far from the laser, the spot size (2R) is big compared to the beam size at the laser output (2r), and it is accurate enough to measure the spot diameter and divide it by the distance, to find the beam divergence.


Example: A laser with beam divergence of 1 milli-radian creates a spot of about 10 [mm] at a distance of 10 [m].

The laser power measured over a defined unit surface area is called Power Density.
Looking at figure 1.8, it is clear that from a laser it is possible to achieve higher power density than from conventional sources (see example 1.2).

This is the reason why a 5 [mW] laser radiation is considered dangerous, and the light out of a 100 [W] incandescent lamp is not !!!