There are several ways i can think of "large volume" concept.
First. Every (official, not alternative) medical text on cleansing enemas that i've ever seen talk about volumes up to 2 liters. Every (official, not alternative) medical shop that i've been offers enema bags up to 2 liters. So that anything above that volume can be considered a "large volume".
Second. There are a lot of recommendations on estimation of cleansing enema volume from weight, age, etc... The simplest one I've found is here https://www.medigraphic.com/cgi-bin/new/resumenI.cgi?IDARTICULO=52147 , assuming formula 0.02*weight_in_kg for pediatric population. There are also different formulas and tables, but mostly for young part of population. So that anything 10% above that volume can be considered a "large volume".
Third. Everything above that person can take easily. This is differs much. For me - easy volume is 1-2 liters. So my usual 3 liters can be considered as "large volume". For my (not very small body) near and dear even 0.5-1 liter is very uncomfortable. For my play partner (who is sensible smaller in body side) 1.5 liter never gives any discomfort.
Fourth. While a usual enema targets sigmoid and descending colon, an enema that targets transverse and ascending colons can be considered large volume. For me the feelings of water in transverse colon starting on 1.5-2.0 liters. And sensation in right colic starting at 2.5 liters.
Taking all of that into consideration, i can conclude that every known to methods are agree on that everything above the 2.5 liters is definitely a large volume enema. And enemas in the range from 1.5 up to 2.5 liters can be viewed as large volume ones depending on a person abilities, tolerance, colon volume etc.