Minimum diameter of particles to distinguish from fluids or gases to be a pellets Pallets collide inelastic and lose mechanical energy and gas molecules collide elastic and don't lose mechanical energy. So which is the exact diameter of each particle or molecule to distinguish both conditions?
 A: Webster.com definition of pellet
a :  a usually small rounded, spherical, or cylindrical body (as of food or medicine)
So it must be large enough in order to hold the molecular structure of  the "food or medicine ...." . 
So you are actually asking how many molecules can make a pellet. 
In this introduction to nanotechnology :

Nanotechnology is defined as the study and use of structures between 1 nanometer and 100 nanometers in size. To give you an idea of how small that is, it would take eight hundred 100 nanometer particles side by side to match the width of a human hair. 

Searching for "pellets in nanotechnology" it seems that they are used,

Dr Eddy Poinern and his team from the Murdoch Applied Nanotechnology Research Group worked with powdered forms of the bio ceramic hydroxyapatite (HAP) to form pellets with a sponge-like structure which were then successfully implanted behind the shoulders of four sheep

Also  here

Using pellets containing nanostructured palladium and gold as a catalyst to breakdown chlorinated compounds contaminating groundwater. Since palladium is very expensive the researchers formed the pellets of nanoparticles that allow almost every atom of palladium to react with the chlorinated compounds, reducing the cost of the treatment.

As it is possible to form pellets using nanotechnology I would take as the scale where pellets can be formed and deformations happen in collisions of the order of magnitude of some of nanometers.
Searching for "deformation of nanopellets"

Quasi-spherical or quasi-rod gold nanoparticles with an average diameter of 3.8 nm are randomly precipitated in a silica-based glass by a heat-treatment method. After ultrafast laser irradiation at 400 and 620 nm, optical absorption, birefringence, and dichroism measurements are performed to investigate the modification of gold nanoparticles shape. Theoretical simulations have been carried out to interpret the experimental results.

So it is an active area in materials science.
If we take 1 angstrom i.e. 10^-10 meters  as the radius of a molecular interaction range, order of 1000 molecules fit into a 1 nanometer radius spherical pellet.  
