They are typically 1/20" or 0.050" keys. Sometimes it's Bristol drive but if someone forced a hex key in there it's probably not a Bristol drive anymore.
Unfortunately I have found no way to extract a stuck set screw of that size. You might try a 1.3mm key, sometimes they really are 1.3 and not 1.27 so you get a bit more grip. I don't know, seems random.
