A Christmas tree skirt is a circular piece of fabric that is placed at the base of the tree to hide the tree stand and catch any fallen needles or decorations. To measure the size of the skirt you need, you will need to measure the circumference of your tree trunk. To do this, wrap a piece of string or ribbon around the base of the tree trunk and mark where the two ends meet. Then, you can use this measurement to find the correct size of skirt for your tree.