Sorry about this foolish question, but I did not succeed in getting the anwser that I need.
In the mesh option of Netgen 3D Parameters, the max. and min. mesh size is always set to be 1000 and 0 as the default, which is also the case in other platforms like FreeCAD and Salome Meca, but I wonder what these two numbers exactly mean, does it mean the volume of the elements are 1000 mm3 and 0 mm3, respectively? If not, what units should they have?