I'm afraid you can't : you need a reference in the real world to have pixel size.

But maybe :

• You can shoot a picture of a ruler or any know size object at the same magnification (if you have access to the microscope of course).
• Or you can compute a micro-marker / scale-bar from the picture if you know the real size of a structure in the picture.
• Or maybe the pixel size is saved in the picture file by Keyence and you can get it from the file header (don't use Keyence, so I don't know).

If there is a scale bar in the image the software must have computed it from a previous calibration. The length should be either given in the image (it might be small print) or in the details identifying the image.