That's an interesting question...but I don't believe there is a specific answer for it. The average colon (large intestines from ileocecal valve to anus) is approximately 5' long with the largest diameter of 3" at the cecum. However, the colon has the capacity to expand considerably. There is a condition known as megacolon (yes, that is the medical term!) where fecal matter is retained for a variety of reasons and the colon can become massive. I have seen pictures of an autopsied megacolon where the diameter was greater than the bicep of a very muscular arm! The colon is also constantly absorbing water so knowing how much is actually contained in the colon and how much has been absorbed would be impossible to say. Everyone is different in this regard.
As toxic waste builds up in the colon, it often layers so that the passage of new waste becomes more narrow. (This passage can also narrow as a result of constictions, spasms or growths in the colon.) Often the stool will be thinner in diameter when it is expelled. It is important to keep in mind that as this build up progresses it can cause entire blockage...but it can also result in the expansion of the colon where some new waste continues to expel while the buildup continues resulting in a ballooning of the colon. Just remember that the colon is not like a pipe that remains the same diameter as it becomes clogged. This is why there can be such an enormous quantity of old, toxic material within a person...even if they continue to move their bowels!
During a colonic the total quantity of water used can be measured, but the amount inserted prior to a release cannot be determined because there is a continuous process of inflow and outflow. Some CHTs will use a counting system to guage 'more' or 'less' water, but not the precise quantity, and people vary greatly as far as their 'count'. I can tell you without a doubt, though, that as the colon becomes cleaner the individual is able to handle more water in the colon before expelling it. A sensitive client and CHT can also determine approximately how far into the colon the water is traveling. The sensation is very different from what is experienced during an enema.