The most significant problem people run into when shopping for a Christmas tree is deciding how tall the tree should be. Most stores will tell you to buy a tree based on the trunk's diameter. While this information is good, you need to know how tall your ceiling is. As a rule of thumb, you should try to keep the height of your tree about a foot shorter than the height of your roof. That way, you won't have to worry about decorating around the top of your tree.