Up until my 50s (I'm now in my late 60s), I used a standard two-quart red enema bag so a two-quart enema was always my maximum. It didn't occur to me that I needed to go beyond that...
...until...
About five years ago when read on Zity HHH or triple-H enemas, which stands for high, hot, and a heck-of-a-lot. That intrigued me, as did the stories and discussions about colon tubes that poke way up inside to get the water "high" in the colon.
I found OptimalHealthNetwork.com with lots of enema equipment, including a few colon tubes and open-top, silicone bags with 3-quart or 4-quart capacities. I bought a 3-quart, open-top silicone bag and found I could take all three quarts so I bought a 4-quart, open-top silicone bag and found that 3.5 to 4 quarts was my maximum, but I also found that it was not very comfortable and I didn't really see the point of trying to max out on some enema-capacity record if it wasn't enjoyable. No problem if that's your thing--have at it.
So, now I just do mostly two, but sometimes close to three, comfortable quarts. Usually the three-quart enema just happens naturally after I've done a two-quart enema. I suppose I'm stretched out a little and relaxed after the first enema and I've cleared out some of the gunk so my bowels can just take an extra quart without a whole lot of effort.
I mostly just fill up with whatever comfortably fills me up, and that turns out to be two and then close to three quarts, for the record.
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Check out SiliconeNozzles.com if you are looking for some interesting and unique insertable items including a huge variety of colon tubes, as in some truly giant colon tubes as long as 100 feet (30.5 m). Oh, wait. Scratch that. Those are garden hoses. The length of the colon tubes can be ordered from 12 to 68 inches (30.5 cm to 1.52 m), depending on the diameter. The diameter can be ordered from .825 inches to 3 inches (2.1 to 7.6 cm). Lengths vary depending on the diameter of the colon tube.
A colon tube with a length of 24 inches (61 cm) and a diameter of 1.35 inches (3.4 cm) costs $101:
https://siliconenozzles.com/products/ct-103french-1-35-diameter-silicone-serpentine-colon-tube?variant=51240056750378
Here's a colon tube with a length of 68 inches (1.52 m) and diameter of 2 inches (5.1 cm) for a mere $345:
https://siliconenozzles.com/products/ct-152french-2-inch-diameter-silicone-serpentine-colon-tube-all-sizes
Have fun.