Obviously, individuals vary, but here is a site that has some average info:
https://bionumbers.hms.harvard.edu/bionumber.aspx?s=n&v=3&id=111127
“large intestine diameter 4.8cm; length 190cm cm”
Assuming the colon is round, the cross section area is about 18 sq. cm. and the volume about 3400 cc.
That number does not seem unreasonable because many people can just about take that in a mighty fill, if they have previously been petty well cleaned out by prep enemas, poo'd out.
An average poo is roughly 1-¼" in diameter, so has a cross sectional area of 8 sq. cm. So, if the person is full of poo, it might be 100 cm long (about 40"), so it's volume is about 800 cc. That allows about 2600 cc of space for water in a petty constipated person… a bit less than 3 quarts.
So, a 3 quart enema should petty much fill an average, pretty constipated, person petty full. This is not really a surprise because the numbers agree closely with what has been practice for over 100 years.