I am not a doctor, nor a nurse, nor do I have physiology or health-care training of any kind. But I do have a fair amount of time-in-grade as an enema aficionado, including retaining medium and large-volume tapwater enemas for up to an hour at a time, and sometimes several sequentially.
I generally find volume of up to about two liters to be pretty much a non-issue as far as my absorption is concerned. I believe that is because my colon comfortably holds that much volume without much stretching of the colon walls.
Absorption problem arises when I increase the volume to the point that it causes significant stretching. While I absolutely love the sensation of that stretching, the stretching occurs because with added volume I have created a significant pressure increase on the walls of the intestine.
No way around the physics. Visualize a large toy balloon; if you blow it up to only the point where the balloon takes shape but the rubber isn't yet stretched, there is no significant pressure on the rubber. As you continue to inflate it, then the pressure stretches the rubber. If I fill the empty space inside my colon with water but don't yet stretch the colon walls, it's a similar situation. Once there is enough water pressure in my colon to significantly stretch the walls, that pressure forces water osmotically into the tissue, resulting in undesired fluid absorption. A little stretching causes a little absorption. A big stretching results in a big absorption. A big stretching over a long time results in dangerous absorption.
I find if I keep my retentions to smaller volumes, and immediately release larger volume enemas, I'm usually just fine. Everyone's colon is different, but I'm just an average size woman and mine will comfortably hold two liters, and I suspect most adults' colons will.
Sorry I don't have a specific answer to the original question, other than this comment that it depends on the pressure.
Hugs,
Diane