particle size

Sugar dissolves faster than a sugar cube, and powdered sugar faster still, simply because the same amount of material is broken into smaller pieces with far more surface exposed to the liquid. Particle size is that same idea for a drug powder: how big or small the individual solid grains are, which quietly governs how the powder dissolves, flows, and behaves.

Smaller particles have proportionally more surface area for their mass, so for a poorly soluble drug whose absorption is limited by how fast it dissolves, milling the powder finer can meaningfully speed dissolution and improve bioavailability. Particle size is usually described not as a single number but as a distribution across a range, since real powders contain a spread of grain sizes; it is controlled by milling, micronization, or crystallization conditions and measured by techniques such as laser diffraction.

Smaller is not always better. Very fine powders can become cohesive and flow poorly, clump together, generate static, or even pose an inhalation hazard, all of which complicate manufacturing. Particle size also has to be kept consistent from batch to batch, because a shift can change dissolution and content uniformity. For inhaled drugs it is doubly critical, since only particles in a narrow size window reach deep into the lungs.