Some of the Hanbalee scholars cite in their books the story that says 'Alee sent his daughter Umm Kalthoom to 'Umar for him to look at her, so he examined her legs because he wanted to marry her. 
Hence they say that this is evidence that it is permissible for the one who intends to marry to look at the sister he intends to marry, dressed in the manner she usually dresses at home, thus he can look at her hair, her legs and her arms.
However, the truth is that this story is not saheeh, and the correct opinion is that it is permissible for him to only look and her face and her hands, and the scholars have stated by looking at the face and the hands he would be able to tell if she is fat or thin. And as for what is besides this, it should be covered, and Allaah knows best.
Shaykh Mashhoor Hasan Salmaan
 Narrated by 'Abdur-Razzaaq in his Musannaf (hadeeth no. 10352)