My husband Scott and I have been hauling our tree on our Xtracycle for about 5 years now -- First on a Free Radical connected to a Specialized mountain bike. When that was stolen, we upgraded to an Edgerunner, purchased through Bicycle Space!
When it comes to picking a tree, we just pick the one we like most. Last year's haul, with me on the bike, was nearly 8 feet! We have the luxury of doing this since our bike is so versatile. We have strap extenders for the Xtracycle bags, which allowed us to secure it to one side of the rear. However, having nearly 100 lbs on one side of the bike make for some tricky maneuvering, as you can imagine, especially when stopping, when all your weight shifts.
What's great about the Edgerunner is that it fits both my husband and I once we adjust the seat post. We've gotten everything on that bike! Kegs, dogs, people, lumber, and of course, Christmas trees!