Should I choose a 2.0mm or 2.5mm derma roller to treat deep stretch marks/scars and sagging skin?

Thanks for your question.  We never recommend dermarollers because it causes unnecessary dermal trauma.  Microneedling, especially with PRP is better because the needles travel perpendicularly to the skin surface.  If you think about the rollers, they go in at an angle, straighten up and come out in the opposite direction.  You end up with long slits instead of tiny pinpoints.  It is illegal to sell medical devices to the public.  These home use devices are dumbed down to not be medical grade (or effective).  Best of luck.