Hollow Carbon rods only come in 20" lengths. If you need a longer endpin, consider solid carbon fiber, titanium, or steel. Size indicates the diameter of the hole in the bottom of your cello for the fitting to be pressed into. If you are unsure of the size, you will need to take your setup down, pull the endpin fitting from your cello, and carefully measure the diameter of the hole. A luthier can do this for you if you don't feel comfortable doing that.