The discussion about height is found in Chapter 1, early in the flashback. Gene says he and Finny were the same height, five feet eight and a half inches. Gene adds that he himself had been claiming to be a half inch taller, until Finny had called him on it:
. . . he had said in public with that simple, shocking self-acceptance of his, "No, you're the same height I am, five-eight and a half. We're on the short side."
Finny won't lie about his height, and Gene understands why: Finny accepted himself exactly as he was, a fact that Gene found "shocking." Finny was secure in his own identity and lacked the competitive nature born of insecurity. In contrast, Gene was very insecure about himself, constantly striving to make himself acceptable, to himself and in the eyes of others.
An interesting feature of the novel is that, of all the characters, only Finny is not given a last name. He seems to represent the ideal, and in his easy self-confidence, he stands in sharp contrast to the other boys at Devon, Gene included.