Funny this thread should be revived now. Last weekend a couple guys in the main group I reenact with got together and mounted our GDFB Halberd heads on shafts. Halberds aren't technically axes I guess, but pretty darn close. We left these at 7ft shafts, which is about a foot longer than the ones our Captain got and mounted last year. I like them alot, this length takes getting used to, but "feels" right.
A couple of the guys in my Reiver group are thinking of picking up some axes to carry, so me and another guy did some checking into them in the 16th Century. From what we found, double headed axes are tools, not purpose made weapons. That doesn't mean they weren't used as weapons in a pinch, but certainly they would have been emergency or last resort weapons. Single headed axes, often backed with a spike or hammer head for balance were the more typical weapon axes in the 16th Century, for early periods I am guessing this was also true. Apparently, though, Reivers sometimes carried double headed axes to chop through those heavy wooden doors found on bastle houses and such along the border. I guess chopping down a door is alot like chopping down a tree, but with people shooting at you and such. Single headed axes of various types were popular weapons because they could double as a tool to chop through a door if needed.
As mentioned before, if you're going for the movie or fantasy look, a double headed axe is killer. Cult of Athena and Therion Arms have some pretty nice ones, historical and fantasy, at pretty reasonable prices.