The worlds largest living Christmas Tree is located in Wilmington, USA. It is an Oak Tree thought to be over 400 years old. This year it has been decorated with 5,250 lights.
Here you can see the tree being lit in 2007:
The tallest living Christmas tree in America is a Stika Spruce Tree at the end of Main Street in Ferndale, California
You can find information on the worlds largest artificial trees and tree displays in our post World’s Largest Christmas Trees 2009