The first big-block V8 displacement offered in the Corvette for 1965 was 396 cubic inches.
Chevrolet added the 396-cubic-inch Mark IV big-block during the 1965 model year. The L78 version was rated at 425 horsepower and gave the Corvette substantially more torque and straight-line performance than its small-block engines. It was also less expensive than the top fuel-injected small-block, helping make the big-block attractive to performance buyers.
The 396 was offered only in 1965. Chevrolet replaced it with larger 427-cubic-inch big-blocks for 1966, which is why Corvette histories often jump directly from the 396 to the 427. The 350-cubic-inch engine arrived later, beginning in 1969, and was a small-block rather than the first big-block option.
Another important consequence was the decline of fuel injection. Only 771 fuel-injected Corvettes were built in 1965, while the new big-block sold in much larger numbers. Thus, 396 cubic inches is the historically specific answer, not 427 or 454.