Larger DPNs are used for circular items in a larger/thicker yarn. Personally, I've never needed any DPNs over size 10, but that's probablly partly due to the fact that I'm not fond of chunky knits in the first place.
For the length, it really depends on what you're knitting. Although I can't imagine needing 10 inch DPNs for anything that you couldn't use a 16" circular for. *shrug*