What is the best size dermaroller for use with PRP on the face?

At 0.25 mm, you are safe. At say 2 mm, you are going into tiger territory - just try that rolling on a piece of paper or tissue to see the tearing effects on your skin. This especially applies if no counter traction is placed. Technique is helpful, but not all the time. Most Specialist prefer micro needling with RF or energy based micro needling with PRP, as this gives the best results, both for acne scarring and for skin rejuvenation. All the best, Dr Davin Lim. Brisbane. Australia. 

Longer needle lengths create more tissue damage, swelling and bleeding which can adversely affect penetration of these serums. Also, rollers create a more erratic pattern of "tears" through the skin rather than the clean, more evenly distributed punctures seen with the motorized micro-needling machines.  We are finding the results are much better, and treatment is less uncomfortable, with shorter needle lengths such as 0.25 and 0.5 mm.In my office, we use the ProCell Therapies system which combines motorized micro-needling with concentrated human growth factor serums made from human stem cells. It is important that bleeding does not inhibit penetration of these serums.