A Christmas tree skirt should be at least the size of your Christmas tree's base, or slightly larger. It should also be wide enough to cover the entire tree stand, and long enough to reach the floor without touching the ground. Some people prefer to use a skirt that is slightly smaller, but I recommend using a skirt that is at least 3 feet in diameter. This will ensure that your Christmas tree skirt will look beautiful and complete, while providing ample coverage for the tree stand.