You would choose the needle size based on the thread size that you're using in the needle. With 70wt threads, I use a 9, 10 or 11, depending upon which is giving me the best stitch. With 60wt threads, 10, 11 or 12; with 50wt, 11, 12 or 14; 30wt, 14 or 16 and with 12wt, 18, 20 or 22, again always depending on which needle is giving me the best stitch.
Hope that helps!
Elizabeth