This is going to be dependent on which size pump you are using.  Penis pump sleeves are available in different sizes to suit the size of your pump cylinder. Each will have its measurements listed on the website, so get one that will fit snugly over your cylinder without any gaps or being too overstretched. If it is too lose you won’t get the right suction and it will slide around. If you go too tight it is likely to tear fairly quickly. There are also universal pump sleeves available that are designed to fit a range of pump sizes. That way you can use the same sleeve across different pumps if you have more than one. The measurements will still be listed on this style so do check them out even if it states that it is universal sizing, especially if you have a particularly small or large cylinder. Standard pumps typically have a diameter of 1.75 - 2.50 inches which most universal sleeves will fit.