I went for a compromise on my A65 bitsa - I used bars for late '60s Triumph twins (U.S.-type). Rise is five inches vs. eight for the stock U.S. BSA bars. They came 32 inches tip-to-tip; I cut an inch and a half off either end to make them 29 inches. I find these bars comfortable, and with adequate leverage.
Prior to that, I had the BSA "home market" (low) bars. With those, I was getting a stiff neck on the 15-mile trip to work on the interstate, from holding my head up. Also, I found it impossible to maneuver on rough terrain with the low bars.
Regarding length, someone here, I think it was Bonzo, said to get the right length, do a pushup, and measure the distance between your hands on the floor.