I actually worked for a local jewelry manufacturing company for about a year, doing crocheted wire pieces... hopefully my experience can help you out...
you want very fine wire, somewhere around 28 gauge. there are a couple factors that will determine just HOW fine you want to go;
first off, the softer the metal, the larger you can get away with... copper is really soft (and now available in pretty color coated variety
) so it's pretty easy to work with (which is good news for your hands). DON'T start with brass or any sort of stainless steel... these are much stiffer wires, and they'll be a pain to work with. Once you upgrade to the more precious metals, try to find "fine silver", as opposed to sterling. It's softer, so again, easier to work with. Gold-fill is less expensive than solid, but it's also harder to work with, so consider using a slightly thinner gauge.
secondly, your needle size will make a huge difference. I typically work with steel hooks between 6 & 0... the smaller your hook is, the more resistance the metal will have to being bent around it. So if you start with a slightly larger hook, it'll be easier on your hands, and you won't have to fight the metal quite so much.
I did find that working with metal (at least in the quantity that I was doing so) was hard on my fingers... one tip I picked up was to get some medical tape and wrap it around your finger (it sticks to itself, so you get a soft kind of a thimble). Otherwise, you'll just have to suffer through till you form a callous
Also... working with metal stresses your hands and wrists, ALOT more than normal crochet. If this is something you really want to work with, learn to take breaks. Stretch your arms, rotate your wrists, stretch your hands... your body will thank you for it in the long run (I never even thought about it when I was doing this professionally, and now I have tendinitis
'course, I was crocheting metal for multiple hours at a time, days in a row, for over a year
Anywho, good luck
If you have any more questions, feel free to send me a msg