I do not know the gauge of the needle used. Whenever I have gone to a donation center, I have found they are very willing to discuss the process and answer questions.
Each time I have donated, they use the inside angle of my elbow. This is a spot where an artery is close to the surface.
Here's a FAQ from the Red Cross:
http://www.redcrossblood.org/donating-b ... ation-faqs
Don't be afraid of being squeamish about the donation process. I certainly am! I am not excited either about needles. I have even told them that I am - and they help me out. Heck, I have fainted
several times, before and after (not on the same occasion), so I have woken up several times on the floor with lots of people suddenly concerned about me. Awkward! But the way I handle that is to tell them "I am a fainter" and, again, they're willing to help out.