There are so many different aspects of a laser that you can measure. Which leads to the obvious question:
“Which aspects of a laser should you measure?”
While there is surely a time and place for all sorts of different metrics, these 3 things cover your bases most of the time:
Why these three?
Power and size, together power density or “intensity,” is the biggest factor when considering how the laser will interact with material. (Good cut or bad cut, strong weld or weak weld, etc.) If you imagine a laser is like a knife, the power density is the strength and sharpness of the knife.
Focus location is important since your ideal beam intensity is probably occurring at focus, certainly not throughout your laser beam. In that case, you better make sure that the work piece is located at focus. Taking the knife metaphor again, ignoring focus location would be like trying to cut with the blunt end of the knife.