Function of all needles size

There are theories about this, but they all come down to a general idea of whether you feel the needle does the work or whether the body does the work in response to the needle. If you, as I believe, think the body does the work in response to the needle it hardly matters what size needle you use - so you generally use smaller diameter needles (34, 36, etc.) as they increase patient comfort generally speaking. If you think the needle does the work you might be drawn to thicker needles (30, 32, etc.). Length, again, is a similar idea, although for most people 1" needles are more than adequate, in heavier folks 1.5" or 2", particularly in areas such as the lower back.

Alot also has to do with your own personal preference, whether you free needle or use an insertion tube, what type of needling you do generally, etc. There are no clear answers to most questions like this in Chinese Medicine as there are many different approaches and theories.