The important factor is to be above the peak of the tent. The higher you go the less chance of sparks and the better the draw.
A longer pipe can help to compensate for a marginal diameter of pipe but only slightly.
The problem with going too high is you create a lot of wind surface area.
I started with a 100" pipe on my Kifaru Sawtooth, because I haden't had time to cut it down. It was fine but way more than I needed. I have since cut it down to 80".
As far as being too long reducing the effciency do to cold exposure, I don't think thats an issue. If the stove is cranking if is glowing red for a good ways aboue the stove top, any temperature difference would be too slight to feel IMO.
