I started with cheap drugstore bags in the 90s, the pink latex douche bags that broke easily, and the 2.5qt hospital style PVC bags. Those were my favorite back then, and the flow was so fast due to the wide barium tubing. Once I moved out, I got a rubber 4qt Klystra that was great. It took me a long time to work up to 4qts, as I didn’t really work on volume training until I moved out.
Years later still, I picked up a JBL Klystra and 4qt Silicone Klystra bag. I could take the most from the JBL, about 4.5qts. For some reason it always seemed easier, as has been discussed here before. Now I have a couple of OHN 5L bags that actually hold a bit more than that, and an OHN ”3L” that actually holds exactly 4L when filled to the brim. That is my “daily driver” and I can take 4L on the first in a session now any time, any place. About 3.5L feels the best to me now though, full without feeling stuffed, which is what I crave most of the time. I’ve taken a bit over 5L before on a handful of occasions, and it’s a different feeling for sure. Have to be prepped right and really clean already, but I think there is something to be said about being just completely filled. I’ve read the stuff around Kellogg and how anyone can take 6 quarts, and I’m not sure if that’s really true — it took years of practice for me, admittedly with that not being the goal 99% of the time.
I also have some ”camp shower” bags that hold something like 5 gallons, and I’ve used those for colonics to great effect, plus a couple of other large ~6L bags designed for water filtration / storage that work great for enemas, e.g. the Katadyn Base Camp Pro 10L.