It doesn't matter in life what you do, from baking a cake, running a race or driving a car. Do it wrong and you can end up with a disaster. The same applies to jelqing. You have to get it right otherwise there’s a very good chance you’ll injure yourself. Pumping can also be of benefit in gaining extra size, though in my view it’s not as effective as other methods and it’s better to pump for fun and accept any gains as a bonus. Having said that, I don’t feel there’s any one aspect of PE that corners the market; it is better to adopt different approaches and use them in tandem. Incidentally, you only need a pumping cylinder a couple of inches or so longer than your cock. While a 14” might work just as well, you may find it impractical.