Well, it should be obvious that the more enemas one has taken, the larger an enema one can take, one's colon becoming stretched and acclimated over time while one also becomes accustomed to the procedure.
For an enema virgin, I say that anything above what is standard nursing procedure would constitute a large volume enema. For cleansing enemas, contemporary nursing textbooks instruct 1 to 3 pints (500 to 1500 cc), although textbooks from 100 or so years ago specified to 2 to 4 pints (1 to 2 L), which I'm guessing because people in those days were more accustomed to taking enemas. However, for barium enemas, for which the patient has painstakingly prepared with extensive colon cleansing, usually five pints (2500 cc) or more are used.
Of course, no one on this forum is an enema virgin! So, while medically anything over a quart or so constitutes a large volume enema, we all have our own subjective definitions of something rather bigger as a large volume enema. As a teenager, I thought of 2 quarts as a really large volume, but now for me it needs to be over 4 quarts to be considered large volume. Personally, having taken thousands of enemas in my life, I've endeavored to slow down this increase by raising the bag ever higher over time, as well as by increasing the intensity of the enema solution. So when I want something "larger", rather than making than using more volume, I will do something like put a bar Kirk's Castile soap in the bag, and leave it in the bag the whole time the solution is flowing into me, which gives a quite a kick. Once I'm filled, if my enema already seems intense then I just lie there, but typically I will get up and do things, usually drinking my morning coffee, and then engaging in physical activity as needed until I simply can't hold it any longer; for a sex -related enema, just "lying there" consists of having foreplay performed on me while "engaging in physical activity" may be mounting my husband.