Make a tail to complete your "Na'vi" character costume that is a custom length to your height. Avatar character costumes that are sold from costume shops have a tail that is one length, which can be too short if you are a tall person. You can make a tail out of fabric of any colour, because you can paint it the right colour of blue.
Measure from your waist line, or wherever you wear your belt, down to 2 inches above the floor. This will be the length of your tail; write down the measurement.
Fold a piece of cotton fabric in half so the "good" side of the fabric is inside.. Measure and trace a tail shape on the fabric with sewing chalk. Make it the length that you determined with a width of 4 inches across at the start gradually narrowing down to a point at the other end. Pin the fabric together within the traced lines.
Sew the two sides of the tail together along one side, about 1/4 inch away from the edges of the fabric. Sew the end closed. Remove the pins. Turn the two sides inside out so the right side is exposed.
Stuff the end of the tail with craft stuffing to make it round. Continue stuffing another 6 inches and pull fabric taut. Repeat this process until you have sewn and stuffed the whole tail.
Paint the entire tail with a thick coat of blue acrylic paint and wait 6 to 8 hours for it to dry. Try to match the shade as closely as possible to the shade of blue of the characters' skin. When the paint is dry, glue the wide end of the tail to the back of a belt with epoxy adhesive and wait 24 hours for the glue to dry.
Add a tuft of dark hair from a craft store to make your tail realistic. Stitch or glue the hair to the end of your tail.