So I have been told on a few occasions that a keli that can hold a certain large amount of water does not require immersion before use. What is the minimal amount in gallons that would make t'vila unnecessary?
What you were told on many occasions is incorrect.
The Binas Adam (85/66) says specifically that holding a large amount such as 40 sa'ah is not a good reason to negate the need for tevila. (There is a discussion there concerning vessels which are attached to the ground.)
The Chachmas Adam there (klal 73:13) is discussing large vessels and he rules they should get tevila.
His words are brought in the first Pischei Teshuva in Yoreh Deah siman 120.
The limud zchus for people who did not immerse the beer vats, which were the center of the discussion, was to assume the opinion that since there was still much prep needed to make the beer drinkable after removing the beer from the vats, tevila was not needed. But the preferable option is to drill a hole in the vat to make it unusable and have a Jewish person fix it.