This is a discussion that I really don't understand at all. While size matters with many things, I have always thought that quality beats out quantity. If I think back to some of my enemas there have been those of just one quart that were hands down better than many of those when the bag or can was filled to the brim.
I second @Emom here that quality beats out quantity. I too for a long time, influenced by reading posts and articles, thought I had to somehow reach that elusive target of 4 quarts. But that is an arbitrary number, and so much depends on one's individual anatomy, as well as many factors specific to the time I'm taking one, key amongst them being how well cleaned out I am, and how comfortable and relaxed I am. Having taken hundreds of enemas over the years, I know that 2.5 litres is what I need to fill up my colon, so I aim for that to flush myself out initially. Only then I aim for 3+ litres, which is the zone where I enjoy the best sensations, as my belly really gets into the extra distension and stretching. Here I just go by feel, stopping the flow if it starts going from pleasurable to unpleasant. I will keep extra water in the bag just in case it happens to be my lucky day, but I don't force myself to empty it if I can't. Sometimes my final volume will be 3.25, sometimes 3.5, and very rarely 3.75 litres. Usually by this point the pressure within starts to resist the flow, and the anal sphincter is also close to giving out (which is why I would ideally always want a retention nozzle for very large enemas, as it allows me to fully relax my muscles and perhaps be able to take an extra 250 ml or more).
But the main thing I aim for with a large enema is not the volume, but how it feels. The stretching, the tautness, a comfortable uniform “ballooning” of the colon without any cramps or pressure spots. Sometimes I've achieved that with a lesser volume, sometimes I end up taking more. Ideally, I'd like to stay with that feeling for as long as possible. After all, it takes so much prior preparation to finally get to that point; no point in ruining it too soon by trying to overdo it and then rushing to expel.