anything from 21-25, 21-23 being optimum to draw... 23 being optimum to inject... but will take anything on a 22-25... though 25 in not prefered as I tend to wiggle to much, push too hard and takes takes too long!!
I use 18s to draw and 22s to shoot. I ran out of 22s and had to use a 18 for a couple of shots,because the pharmacy refused to sell me any. Actually wasn't that bad.