While some people have done wild engine swaps (and some of those are documented online), a truly *practical* engine swap would involve significant fabrication, custom mounts, drive shaft modifications, and likely changes to the suspension and possibly the frame to handle the increased weight and power. Even then, many bigger engines simply won't fit physically.
Therefore, there's no single definitive answer to "the biggest engine." It's a question of engineering ingenuity, budget, and how much you're willing to modify the car. You'll find a wide range of swapped engines documented, but there's no upper limit except for physical constraints and the builder's limitations.
