The tale "The Smallest Dragonboy" has Keevan, a boy who is shorter than the other candidates (and is teased about it relentlessly). Keevan's always been a good student during his training sessions, but his size downgrades his chances. Keevan is badly injured by his arch rival (and all around Jerk Ass) Beterli. While he recovers he's assured that he will have other Hatchings. Despite everything, Keevan manages to find his way to the Ground and Impresses a large bronze dragon named Heth.
He shows up in one of the later novels as a capable Weyrleader, and apparently more than caught up to his peers in physical size.