It's best to start with one that gives you some stretch - but is comfortable, and then after a few months when you've lengthen your sack, you canmove up to a longer one. Many also have small fasteners or rings from which you can hange addition weight. The longer your wear it, and the more time you're able to comfortably have the device on the better results you will see. This is yet another way to get a looser ball sack. Using a vacuum cylinder and pump you can pump the testicles up to some phenomenal sizes and lengths.
Some men pump both testicles and penis at the same time.
If you really want to get them up to freakishly large sizes you can get a custom made two stage tube . These tubes are custom made in the size appropriate for you. They are called two stage as they have two different sized chambers. The first part goes against the body as you slide into the tube. This part is kept to a smaller size as it must be able to form a seal against the body.