What units are the hair particles' diameter are measured in? I'm creating characters in Blender and using the hair particle system for the hair. When I use the particles, the strands sometimes render too thin or too thick. I saw some tutorials on YouTube and understand some the basics. I done some research online and not all hair share the same the diameter. The real world units for hair diameter measured by micrometers, something that is not clearly shown in the particle settings. I want to know what units of measurement that the hair particle system uses or would that be overkill to get such a measurement?
Cycles hair particle size units, like most other settings in Blender are expressed in Blender scene units.

The Scaling parameter is the actual width of the hair in scene units. The confusion might stem from the fact that on top there are scaling factors applied that may induce doubt.

Setting a Scaling value to $1$ and leaving the Root at $1$ and Tip at $0$ will yield a hair particle of length $N$ (set in the length property at the top) that is exactly $1$ Blender unit wide at the root, and progress to zero width at the tip.

Tip and Root are scaling factors applied to the base Scaling property, having a root value of $0.5$ over a Scaling of $1$ will lead to a root of $0.5$ Blender units. Likewise having Tip at $0.2$ will yield a hair particle with $0.2$ BU at the end.

The Shape property will control how the width shape progresses from root to the tip.

Here we se a single hair particle with $1$ unit of lenghth applied over a $1\text{ x }1$ plane, and starting with a Scaling of $1$, Root of $1$ and Tip at $0$.