Units inside the computer are completely arbitrary. It is you that defines the measurement between two points.
This means that you can determine the units you need. ZBrush then provides the tools you need to work within that. Simply draw a TransPose line between any two points where you know the distance between them. Then use Preferences >> TransPose Units to tell ZBrush what that measurement is. From that point forward, any TransPose line you draw will utilize those measurements.
You can also import a grid to serve as a background while you work. In this case you would draw your TransPose line between two points on the grid and set the value using the TransPose Units sub-palette.
The important thing isn’t so much the units as that everything you sculpt be proportionally accurate.