I'm not a fan of large nozzles. The first few years when I was starting out, even the infant rectal pipe was big enough to fit without leaking (I was about 14 at the time, and very much a "late bloomer" -- didn't even sprout any pubic hair 'till the following summer.
As I finally began to catch up, the regular adult nozzle now gave a comfortable fit. I had older relatives, with whom I occasionally stayed, who were into enemas and knew I somtimes took one, self-administered, while visiting, and I knew they used the douche nozzle, but I wasn't ready for that,
I had my first professionally-administered colonic at the age of 32, and the nozzle was roughly the size of the vaginal pipe my relatives had used. It taught me to relax, but I'm still concerned not so much with the size of the pipe as whether it's tapered -- larger at the "business end" than the base, and will stay put once inserted.
One of the most comfortable nozzles I ever owned I found in an old drugstore while travelling some years ago, It was about 5 inches in length, tapered, and had three medium-sized outlets, 120 degrees apart about a quarter of an inch below the tip, which was capped. The holes were small enough to "homogenize" the glycerine I often add to an enema to enhance the evacuative effect.