Cylinder sizing is extremely important. Too small and you either can’t fit inside it or your expansion is restricted.
Too big and you can’t get a good seal and/or your scrotum gets sucked in, which isn’t fun.
You can size a cylinder to be more aimed at length, so you expand fairly quickly in the session to ‘pack’ the tube and thus have more elongation, or you can size for girth where you have more room to expand. Some guys have a staged approach, where they start in a smaller diameter tube and then move to a wider one once they’ve expanded some.
Pumping can be really valuable for increasing size and EQ. It can also be done poorly, with too much edema in the skin, petechiae and bruising of the skin and blisters that result in having to take breaks from PE to heal or cause ED.
I suggest you spend some time reading about it on the forum here to get some idea of what you’re getting into before you purchase something that you end up disappointed with. It’s worth the investment in a little time to get it right the first time.
There’s a whole section of the forum dedicated to pumping. You can start with this, though:
Selecting Your Cylinder Size