How to add scale in DSLR plus microscope set up by default?
Select the objectives you want to use on your microscope altogether with your camera.  If the camera is not a permanent installation, ensure that the mount reliably establishes again and again the same position and alignment in respect to the optical axis of the microscope.

See if your group has a stage micrometer, a slide with fine regular markings:

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If not purchased altogether with the microscope (one other source), or lost during a move of the group you work in, see if an other group in your school active in microscopy may lend you one.  It may be in geology, but equally in (analytical) chemistry, biology, histology, (laser) physics / optics, etc.

Identify a reliable protocol to get hold of the images recorded by your camera to be fed into a suitable software e.g., ImageJ / Fiji.

Take a good photograph with your camera mounted on the microscope observing the micrometer plate, starting with a low magnification.  For once, you have to count the pixels to calibrate profiles for a given combination of objective, inner optics, and mounted sensor / camera. But today, you may get assistance by the software's plugin (example, documentation, tutorial video).