640mm and 36cm if we're measuring from bony landmarks rather than muscle (which I assume we are, and not that my chicken wing delts add much anyway).
My jump bike came with 710s which are likely wider than I need, but I never cut them because it's not like there are any trees to hit where I ride it, and I occasionally find myself grabbing them way out on the ends anyway. Wouldn't want any shorter than the 640 though.

I had barends on my bike when I was in college and lived on a fairly steep hill. Then I actually started mountain biking and got rid of them. Wasn't using them, and just seemed like another pointy thing to impale myself on.