Some people on Facebook set their profile picture as 'Only Me' where no one except the user can see that picture in full/original size. Other users can not see the image in full size except a small view can be seen.
Here is a trick to see private pictures also in Original size as it is.
Follow these steps:
1) Log in to Facebook and go to the timeline of the Person whose profile picture is in private mode.
Right click on the profile picture and click 'Open Image in New Tab'. The picture will be opened in new tab(Small Size).
2) Observe the address bar where you will find s160x160 remove it and hit Enter.
3) Now You can see that the image is in it's Full/Original size.
Also Try this: Hack and Edit Facebook Login Page using Browser.