The Digital Microscope Suite has different versions that are supplied with different Celestron microscopes.
The measuring tool in the software relies on a fixed distance from the specimen and therefore known magnification for its calculations. If the software is used with a fixed configuration microscope, it therefore has no problem calculating the size of an object in the field of view
Since the Digital Microscope Imager is only a camera, the magnification depends on what microscope is used with the imager. There is no fixed configuration. So the simple tool in the software cannot be used to calculate a size for a specimen, and software shipped with the imager does not have this measuring tool.