I have been performing medical microneedling using manual rollers for about fifteen years and Dermapen for about 8 years. Although different areas of the face may require differing needle lengths, the usual range for treating atrophic (indented, depressed) acne scars is between 1.5mm and 2mm. So, I really don't think there should be any problem here. That having been said, I would like to reiterate a point I have made in many of my answers on RealSelf that microneedling is best reserved for treating tone and texture problems that remain after other methods, such as subcision lifting, with or without the addition of fillers, for example, have been used to elevate the depressions. By itself, microneedling, whether manual or automatic, is not the best approach to treating any, but the most shallow acne scars. Be sure you are being treated by a board certified aesthetic physician with experience and expertise in all forms of scar treatments. Best of luck.