The choice between 4x6 vs 5x7, depends on how prominent you want the photograph to be. As Visuals Clipping points out, a 5x7 print will give you a larger space to view the portrait, and that space might make facial features and other items stand out more. The 4x6 print, on the other hand, is smaller and easier to fit into albums or smaller frames. These can be effective for portraits, so spaces available and the impact you are looking for are helpful things to consider.