Simple answer, shorter than your ceiling.
Seriously, start by measuring the room’s height that you are putting the tree into. Then make sure that there are no overhead or nearby obstructions like lamps or fans. That is your rough working space. Next, consider the height of your tree stand and any decorations that you are planning on putting on top of the tree.
Practically speaking, how many feet of lighting do you actually have to work with? It would look awfully silly to have a tree 9 feet tall with only enough lights to cover half of it.
I think you probably need some room for the lights to bounce gently off the walls and ceiling to create a subtle indirect lighting effect. Give the top of the tree at least a foot or two from the ceiling to accomplish this.
NOTE: I’m not a professional Christmas tree designer, so, do what makes sense to you. If you do have ideas or tips that you would like to share, please hit the “Contact Us” menu item and let me know.