Oh, you really don't want to get me started on this... I am a crank on the subject,
But shorter is better. There just isn't any advantage to longer cranks. People talk about leverage; yes, you have more leverage with longer cranks. That's great, but you don't need leverage. You need to spin. And you will spin better with shorter cranks.
Since the shortest C&V cranks you can find are 165 mm, I recommend 165's. If you can find shorter ones, go for it. You won't regret it for more than a half hour or so.