What features should I look for in circular knitting needles to make them easier to use with my arthritis?
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.