Under a Mesh Part node you add nodes for importing and modifying an externally generated mesh.

The Settings window for a Mesh Part node contains the following section:

For mesh files that are unitless, the physical size of an imported mesh is independent of the length unit of the mesh part. For mesh files that define a unit, the imported mesh is scaled to match the unit of the mesh part, if the Use units check box is selected. You can also select this check box to use the length unit and angular unit (which you specify using the Length unit list and Angular unit list, respectively) in the nodes appearing under the Mesh Part node and to specify the length unit of the resulting file when exporting the mesh to a file format that supports length units.