Why does Finny refuse to lie about his height?
Knowles creates Finny as a separate and distinct personality in the novel, unlike Gene, Brinker, and Leper in several very significant ways. It's not accidental that of the four boys, only Finny is not given a last name. This obviously sets him apart from the others. Finny's is idealized through Gene's memory, and as a literary character, Finny functions as a symbol in the novel, a symbol of youth and innocence.
Unlike Gene, Brinker, and Leper, Finny is secure in himself and his own identity. He is not at all competitive. He does not have to be bigger, better, smarter, faster, or more popular than anyone else. He does not have to lie about how tall he is because he accepts himself for who he is. He does not live in fear that he will somehow be less acceptable than others.
Gene and Brinker are slaves to competition. Leper is so insecure and socially inept, he withdraws completely every chance he gets. Finny, however, just glides gracefully through life at Devon, happy with himself and at peace with everyone else. The truth of Gene's betrayal and Finny's fear of being left behind during World War II are the only forces that destroy his security.