Here are two ways to help keep your balls out of the cylinder when the cylinder might be just a little large.
If you’re new to pumping, you should know that it helps to shave or trim at the base of the penis. Honestly, trimming is just about as effective and avoids the potential problem with ingrown hairs.
My initial impulse with pumping was to place the cylinder over the shaft so that the shaft is centered in the cylinder. Add a little lubricant to your shaft, and that positioning makes it easier for the balls to get pulled in when you start pulling a vacuum.
So: trim a little higher than the immediate base of the penis, and position the cylinder so that the lower side of the cylinder is at the bottom side of your penis, with ALL the free space in the cylinder on the TOP side.
This is a technique I still use today. I have two cylinders now, and I do staged pumping sometimes. My balls are in no danger with the first cylinder, but the larger cylinder could still pose a problem if I am not careful enough with positioning and vacuum level.
The second thing you can do to help keep your balls out of the cylinder is to keep the pressure low. I do the majority of my pumping at 2 or 3 in.Hg.