Actually, if you look at the manga, Ranma as girl is about as tall as Akane, who is pretty much average heigh for her age. She's just much more shapelier than average teen.
Plus Ranma can do Hiryu Shoten Ha instantly, this has been used twice: #29 against Happosai and Rakkyosai, in #33 mirror-Ranko (well, girl-type anyway) uses it against Soun and Genma.