We don't know what she looked like, but we are informed by people close to her that she was an attractive, well proportioned girl
Since I can't copy/paste from the source, and some people are having trouble reaching it, page 24 contains the sentence,
"The person closest to her from April 1429 to May 1430 claimed he was surprised to never have been tempted to approach her, 'although she was an attractive, well proportioned girl'. The footnote from that sentence is to a quotation in french quoting DuParc
The book in question is "Joan of Arc: Reality and Myth By D. E. H. de Boer, Marina Warner"