To unlock the It was THIS Big trophy in Red Dead Redemption 2, you're going to need to catch a humongous fish. Bear in mind that you need to progress a reasonable way through Chapter 2 to unlock the fishing mini-game, available after completing "The Fisher of Men".
While you can easily scout out legendary fish areas and other spots in the world to nab this in free roam (make sure you're going after lake fish and using a lake lure for the best results), if you're willing to wait you can track down a stranger called Hamish in late Chapter 5 or early Chapter 6, near a lake called O'Creagh's Run in Grizzlies East. Complete Part I of his mission strand and start Part II, where you will certainly unlock this trophy as
You can also easily track down Legendary Fish after speaking to the Stranger called Gill at Gill Landing in the Heartlands from Chapter 3 onwards. He wants you to catch them all.