How does the narrator know the bass on his line is very large?

The narrator knows the bass on his line is very large because of the immense weight and resistance he feels as he reels it in. The fish pulls with powerful, determined force, making it difficult for him to gain control. Additionally, he observes the line bending significantly, indicating a substantial size and strength of the fish. This combination of physical sensation and visual cues leads him to conclude that it's a large bass.