There are several ways of computing these metrics. The main difference is what is the representation used (surface or volumetric) and if the entire segment is used or only the part that overlaps with the selected scalar volume.
Also, is there a way to compute maximum/minimum 3D diameter with any of the statistics modules?
Currently diameter computation is not offered by any of the Segment Statistics plugins.
SimpleITK’s LabelShapeStatisticsImageFilter can compute many kind of diameters and it is already available in Slicer. So, it would be just a matter of adding a few ten lines of code to run the segment’s labelmap representation through this filter and add outputs to the table. Would you be interested in working on this?
It should be back soon. You can monitor the status here. Maybe add a note there to show that you would like to get this up and working again (and maybe offer your help, if you can).