Seems like 0,5-1mm needles are mostly recommended on forums like these. However, since the thickness of the skin seems to be different at the scalp (thinner in the temples, thicker on the crown). Skin thickness also differs among individuals. Clearskincare (Novaderm) makes a 0,765 needle roller, while nanogen makes a 0,5mm and a 0,3 mm, the latter one made especially for temple areas. It is confusing!!!! It also seems dangerous to use to long needles, since the topicals can enter your bloodstream. But too short needles will not give much effect... I personally can´t decide between a 0,75mm and a 0,5mm, but I am leaning towards purchasing a 0,5mm.