An STL file can describe a triangle at coordinate 20, but it does not say whether that coordinate means 20 millimeters, 20 inches, or something else.
That missing unit is why a structurally readable mesh can arrive 10×, 25.4×, or 1000× away from the size someone expected. Import dialogs and slicers can apply a unit assumption, but the STL bytes do not provide an authoritative answer.
The safest first step is not to guess. It is to expose the consequences of each assumption and confirm one known physical dimension elsewhere.
One mesh, five possible dimensions
UnitMesh is a local binary/ASCII STL preflight. Its deterministic four-triangle fixture has raw coordinate dimensions:
