Surface and volume of an STL mesh
STL file

One of the special features of the STL format is that it is dimensionless: the STL file contains no scale or length information. This means that the file contains no internal description of the units used to create the 3D model when it was generated. The units associated with the dimensions are therefore those you wish to assign to them (microns, millimeters, inches, centimeters...) when you re-open/read the file. Once this has been done, it is possible to calculate the surface area of the facets making up the mesh, as well as the volume included within the mesh.