Don't get too fussed about when you purchase your Christmas tree will easily last 3-4 weeks if looked after properly. Be careful about buying the biggest tree you can find, they are heavy, hard to handle, and you'll need a big, sturdy stand to keep them in. A 6' tree looks great when it's decorated, a 7' tree is wow! If you want it higher, raise the stand and put all the presents in the gap under the tree.
If you are buying a cut tree, the branches will be folded in. You must cut 2-3cms off the base of the trunk to break the seal. The branches will come out once the tree is in water. Lots of people like a really bushy tree, but keep in mind the decorations look much nicer if there are some gaps between the branches.
The tree should smell of fresh pine, and look for nice sharp pine needles. Each tree has it's own shape and character. They don't have to be perfect shape to be the perfect tree. Whatever you choose, the decorations will make it beautiful.