The ideal Christmas tree size for a 12-foot ceiling will depend on the available space in the room. Generally, a 6 to 8 foot Christmas tree would be an appropriate size for a 12-foot ceiling, but you may want to consider a 9 or 10 foot tree if the room is spacious. Ultimately, it's important to ensure the tree is proportionate to the room size, so it doesn't overwhelm or look too small. If you're still unsure, consider consulting with a professional decorator or landscaper for expert advice.