Lower number means broader needle. The needle should be as broad as the thread to not damage the fragile thread. Size 22-20 for Worsted 20/2 and size 18 to heavier threads 16/2, 18/2.
The needles are made of hardened steel with a symmetrically punched eye. The finely ground tip and the smooth coated surface of the needles allow them to slide effortlessly through any material. A balanced ratio between length and thickness gives the needles their good elasticity.