Define size

It is difficult to define the "size" of the powder

So far we have casually used the word "particle size".
However, as most of the world's powders are irregularly shaped, it is (really) impossible to talk about them without a definition of what the word size means.

To complicate matters, there are various definitions of "size".

This is the same way that there are many different definitions for the size of the human body.
Height, chest circumference, or a combination of several values such as length, width and instep height of the foot, weight...
This is because the value you need depends on what you want to express with that number and what you are using it for.
If you want to show the height of a person, you need their height; if you want to determine the size of a water book or a skirt, you need the chest circumference, not the height. If you want to make a shoe, you need the length of the various sizes of feet.
And to determine the load-bearing capacity of a chair, you do not need information on length, but you do need weight.
Thus, it is important to define whether the reference value is length, weight (mass) or some other value (volume, surface area, etc.).

In addition, the shape of powder differs greatly from that of humans. Therefore, the question is: where do we measure the length? This is a problem.
In the first place, how do you measure so many particles (at least a few hundred, often tens of thousands or more)? The problem is that the task of measuring length, which is not so difficult with large objects, becomes a daunting one.

Moreover, even if you are measuring the same thing, the results will vary depending on the method of preparation, the principle of measurement and the measuring mechanism (method).
This is a long story that could be written in a book, so I won't go into it here.
For more information on particle size measurement, please refer to the websites of the manufacturers of particle size analyzers.