I have moderate arthritis in my hands and find that large needles tend to make my hands cramp. This is also true of smaller needles. I find the sizes between 6 and 10-10.5 the most comfortable. (Takes me forever to knit socks with 3's)
I use circs for almost everything because they can be rested on the lap and not cause extra tension on your hands.
I also have to hold the yarn an entirely different way than anyone I've ever seen.
As for length of circs, it depends on the project you are doing. I use the 16" or 24" for hats.
Hope this helps and GOOD LUCK!!