What features should I look for in circular knitting needles to make them easier to use with my arthritis? I have arthritis in my hands and the smaller length needles are a little hard to hold onto. Would longer needles help, if they exist? Is there anything else to consider, such as needle material (wood vs. metal vs. plastic) that would make gripping the needles easier?
Longer needs can be helpful for this.

Addi makes a set of interchangeables with 5.5" (approx 14cm) tips (most interchangeables have 5"/12.5cm tips). They are called Rocket Lace Long Tips.
http://www.addineedleshop.com/click_system/lace_click_long.htm

I found the same set on Amazon by searching for "Addi long lace tip circular knitting needles." Amazon also has these needles in fixed circulars. I would suggest starting with just one fixed circular or one set of tips and a cord before buying the full set to see if half an inch is enough extra length to make a difference.