It’s not really the size of the scorpion (though the smaller ones do tend to be disproportionately venomous) but the size of the tail/stinger relative to the claws. Big claws and little sting is more likely to be less venomous, or at least have less dangerous venom.
Basically if it looks like it can rip something the same size as it to shreds with its claws, that’s usually what it will be doing rather than stinging things; in which case the sting is mostly a defence against larger predators and more likely to be painful rather than deadly.
If it’s the other way around the sting is going to be the primary weapon and could well be quite dangerous and even deadly.
Also, scorpions come in all sizes and are still Arthropods, so they have multiple life stages with moults in between, which makes a positive identification really difficult since whatever just stung you could be an adult or an early stage of something else or anywhere in between.