Allen Key description and size
This specification for the Allen Key included in the Steam Crave Aromamizer Supreme is directly from Steam Crave.
The Allen Key is 1.27 mm.
According to the conversion website, this is equivalent to 0.05 inches, or in fractional inches 1/16th inch.
I've managed to strip the head of two grub screws in the atomizer. A tip I can offer is to make sure the Allen Key is inserted all the way in, and keep pressure pushing in towards the grub screw while you tighten/loosen the grub screw.