In nature, scorpions can be small enough to fit on your fingertip or grow to a full eight or so inches, but not all scorpions are natural. The giant scorpion, of which there are many species, can be found in any area suffused by enough magic to alter reality.
Without magic, a scorpion, or indeed anything with an exoskeleton, could only grow so large, as the weight of that carapace would prevent it from moving after molting. It’s one of the reasons the largest arthropods (like giant crabs) are found in the oceans, rather than on land, where the water prevents them from collapsing in on themselves.
But in highly magical areas, all the normal rules have been relaxed (or in some cases completely forgotten). The giant scorpion is one of the more common creatures found in dungeons and ancient ruins, one of the many reasons to avoid such places.