Until recently I have been using a Klystra KP2 nozzle 1.5" diameter at its widest which felt nice inside and stayed put while taking on water. However, i was beginning to have more and more trouble with hemorrhoids, so I bought a Mikacare enema tubing replacement kit from Amazon which came with several small nozzles including a 3.5" long tapered nozzle that is only 0.5" max where it joins the tubing. One has to hold it in place when taking on water, but it is not nearly so hard on the hemorrhoids. In the past I could take on 2 quarts of plain water in about 2 minutes and hold it for another 5 minutes or so until the urge for release became too much. Today with the Mikacare nozzle I took a little over 2 quarts plain water in about 6 minutes and held it for another 10 minutes. During the 10 minutes the urge to expel increased slowly but was not overwhelming after 10 minutes. It was really nice to retain the water for the extended time.
BTW I bought the Mikacare enema tubing replacement kit because I didn't want to disrupt the Klystra nozzle, back-flow preventer & tube assembly that I had in case I wanted to go back to it. I wanted to cut the old tubing at the bag where the loss of a bit of tubing would not matter. I wish someone would market a good quick-connect, quick-disconnect coupler that would enable one to easily change nozzles. I've found something that might work at https://www.coleparmer.com/p/barbed-straight-quick-connect-couplers , but one needs to buy a package of 20. If a vendor like Mikacare would include one in their replacement kit, it would not add much to the price of the replacement kit.