Choosing what focal length to work with is part art, part science. The basics are these:
First, objects that are closer to the camera will appear much larger in relation to things further back when shooting with a wide angle lens. Notice the distortion on the leg, for example. That distortion is accentuated the closer a part of the body is to an edge of the frame.
This is further influenced by the angle at which the camera is in relation to the body. Pointing the camera down, for example, will make lines at the edge of the frame bow out. Just look at the effect it has on the windows and drapes.
This is an excerpt from my March 2014 monthly email. Sign up for the free fine art nude photography newsletter in the sidebar to the right, or at the homepage for NudePhotoWorkshops.com to see the full article and additional images.