As this is a site to discuss kinks, enemas as discussed here are infrequently medically-motivated, but some are. Mostly we ask and answer queries and discuss enemas either just for anal/rectal/abdominal sensation fun or for sexual arousal. Even if your doctor gives you one or tells you to do one yourself in preparation for a procedure, you'll likely be signing a consent form informing you of the risks, typically including many items from benign all the way to permanent injury and death.
So as with almost anything else, sexual or otherwise, a first rule of thumb is that you're responsible for whatever happens to you, however uncommon and whether you know all the risks ahead of time or not. Thus a common saying in these matters: Have fun and play safe (or at least as safe as you are willing to risk).
I don't know the largest documented colonic or enema ever known, but in 5 years here I've seen claims very commonly from 1-4 quarts of enema, my own demonstrable maximum of 5 quarts (done only once), and a few members stating they've taken on up to 6 or more. I think I recall someone mentioning 7-8 quarts once. All claims subject to documentation. What post #2 below mentions about practice to increase volume as well as the contents of your colon make good sense. What volume enema you can safely and comfortably hold is multifactorial and not even constant between days or weeks.
And volumes change for various reasons. I started trying enemas at age 15 at about the usual 2 quarts in a drugstore bag and liked it for several decades. After joining Zity in 2015, I read a lot about it here and gradually increased my preferred volumes to about 3.5-4.5 quarts with one single enema in which I took 5 quarts as an experiment. Enjoyable and no problems.
As for your own practices, if you feel like trying higher volumes, do a lot of reading, ask questions, increase volume slowly over days to months, don't ever do it to compete with anyone else for any reason, and pay attention to your body. If you feel something isn't right, stop and evaluate, and approach that particular volume with care if there's a next time. For frank pain, same rule - stop ASAP and evacuate and approach that volume more gingerly next time.
Have fun and play safe.
Eric