At the moment I'm using 3.25mm with sock weight merino wool. I think I'm going to try going down a needle size or two and see if I do better with that.
As for length, just the normal bamboo DPN length.. 7 inches maybe? Working in the round, my stitches are kind of going back and forth, too loose and too tight.
I've done lots of stuff in the round so far with no problem, all in sport or worsted weight. I even just did a small swatch practicing a cable pattern in this troublesome sock yarn and it looks fine! All the stitches are nice and even.
You're right on the border of too many questions, one more and my brain might have exploded. :D