What size needle do you recommend when dermarolling at home? I'm trying to save up money to do it professionally.

As above for safety- any longer can cause micro tears in your skin. I have seen some bad scarring from longer needles. Keep traction and pressure even and do not over do it. The use of PRP of EGF can help, new stem cells replacement can also accelerate healing. All the best, Dr Davin S. Lim. Brisbane. Australia. 

Micro-needling is most effective in improving the appearance of the skin by enhancing penetration of topically applied serums, not by the action of needling by itself. Micro-needling creates micro-channels in the skin, dramatically increasing penetration of products by up to 300% compared to normal topical application. Longer needle lengths create more tissue damage, swelling and bleeding which can adversely affect penetration of these serums. We therefore use needle lengths of 0.25 to 0.5 mm in my office. We are finding the results are much better, and treatment is less uncomfortable, with shorter needle lengths. 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.  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. Our patients also use growth factor serum at home after the office treatment.