Because this information is nearly impossible to find, I’ve made a list of Soviet watch glass sizes as far as I’ve personally replaced and confirmed them as fitting. Without this information it’s often necessary to buy 2 or 3 different sizes to find the one that fits. Even with this information there is sometimes 0.2mm variation in the same model, so take these sizes as a starting point.
Watch crystal sizes are commonly listed in units of 100µm (1/10th of a millimeter) so for example size 312 is 31.2mm. The term “crystal” or “glass” is conventionally used even though the Soviet watch lenses were always acrylic.