You can really make a tree skirt of any size - head to the next step for more info on that. You'll need to increase the amount of fabric and binding as needed. :)
If you're not the best at sewing or want to get this done quick, you can easily make this no-sew by employing a hot glue gun or fabric glue. If you go the fabric glue route, I really recommend Beacon Fabri-Tac
. I've used other fabric glues and they just don't hold up - but Fabri-Tac is wonderful. Dries and adheres quickly and stays nice and strong even after washing!