The old rule - place the drop ends against the balls of your shoulder. Exact fit? Perfect. For me, that was 38, 39. I now rode those on my road bike, 42s on my road fix gears for climbing leverage and 43+ for my mountain climbing setup for that fix gear. (The wide bars are also very nice for spinning 200+ RPM coming down!)
Any time I have to go upwind, I really appreciate those narrow bars. Back when I raced, being narrow also had real advantages moving up, etc.
Ben