Ranko is what Ranma calls himself when his mother is around to keep her from
forceing him into ritual suicide, due to a pact Genma made with her to make
him a 'man among men.'  It has been said often in the Manga and the Anime that
Ranma's female side looks alot like him. But there is no 'Official'
exsplanation for who she was\is to my knowledge so just go with what is best
for your story.  (If you haven't seen the Oav 'like water for Ranma' I suggest
you do because that is what the first episode is about. The second story
Introduses another Character into the series 'Miss Hinako') As for her
measurements...when she wears Akane's clothes "the chest is too tight and the
waist leaves room to spare."
I hope this answers your Questions.
do itashimashite
Catbert