Depends on the model, generally the better ones will be more verbose with how they function, but generally their stated magnification is for an assumed monitor size, say a 21" 1080p screen or something, and you can kind of work backwards from there.
Since the focus is widely enough variable (a nominal 200x magnification microscope can often go down to 50x or 20x just by backing it out) and the magnification number is also dependent on the displayed image out of it, they probably should just specify a nominal pixel size at the rated magnification, but that doesn't seem to be a priority on the cheap end.
For more expensive options, like Dino-Lite's microscopes, they specify X and Y dimension field of view for a given rated magnification and working distance, so you can resolve the width of a pixel just knowing the sensor resolution. There are also a number of microscopes that have companion applications that have a measurement tool, so if they can read the focus adjustment, they know how large a pixel should be when in focus.