I do not think that there exist any standard for that. A owder sample normally has particle size of the order of micron but you call nanosize paricles as powder as well. The microsize particles give good X-ray neutron diffraction peaks that are resolution limited whereas nanosize poders show broad peaks.

The question of the type: "What is the crowd? Two, three, four...? 

 There is no precise answer. Any sample can be considered a powder if its properties are controlled by the grain size and the grain surface layer volume is a significant fraction of the grain volume 

Sergei, try to read this

http://www.sciencedirect.com/science/article/pii/0378437189900356

I happen to know Anita Mehta.When she was young, she was very-very beautiful....She subsequently wrote this book   http://www.amazon.com/Granular-Physics-Anita-Mehta/dp/0521187990/ref=sr_1_1?ie=UTF8&qid=1433658138&sr=8-1&keywords=anita+mehta     I believe, this answers your question in full.

There is no standar for particle can be called by powder. I think Powder is not quantity size but quality size. CMIIW

Granular Physics (physics of sand piles) consider macroscopic particles. If the paricle sizes gets smaller than micro then their equations become less and applicable because of the onset of Brownian motions. If the paricles get even smaller and become nanosize then what happens? Nobody knows exactly. This is an unexplored field as yet. On the other hand in common language we consider powder to be mostly close to nanosize. The famous talcom powders used by ladies and aristrocrates in Europe in fashion golden age (hopefully girls do not use them any more and the so-called powder rooms do not exist) were certaily fine enough to be nanosized. They of course did not think of the hazards of nanosize for lungs etc.  

Thank you all for your response and I apologize for the vague question.

I mean something like this. For example, if the particle size of the small molecule the ansemble is not a powder. If the particle size, for example 3 mm, this is not called a powder. Thus it must be some size range to be adequate, the term powder.

The nanosize particles are powders too.

Micro- and nanosize particles with narrow size distribution (small PDI).