People who get mad when....
the actors cast in a film adaptation of a book they read don't match the book's physical descriptions are, to put it bluntly, idiots. PLEASE tell me where you all expect to find an actress who is:
A. incredibly talented and capable of portraying an extremely complex and psychologically disturbed character with the skill of an absolute pro
B. a big, recognizable name that can realistically capture a return at the box office
C. Less than five feet tall with red hair and enormous boobs?
You need a reality check if you think it's some kind of outrage or detriment to the film to cast an actress who doesn't closely resemble the book description. Anyway, the small stature of Libby Day is used in the novel as a poetic literary device to represent her inner feeling of vulnerability. Barring the use of some horrible forced line of dialogue, this metaphor would be completely lost on screen either way. Furthermore, that sense of vulnerability is something a truly gifted actor (like Theron) can portray with or without being physically small.