http://techcrunch.com/2012/03/22/fac...os-fullscreen/

Facebook used to limit the viewing of photos to roughly 3/4 of your computer screen. If you're using Firefox or Chrome browsers, now there's a way to view photos in full screen mode. Click on the expansion arrow in the top right of the photo.

Facebook still limits the maximum size of photos you can upload to 2048x2048 pixels.