it depends on what you intend to use it for if your going to down load a lot of disc based games go with a large size if you are just going to use it for dlc and smaller e shop games 500gigs should be plenty. I got a 3tb one but I figured I can use it for other stuff if I need to.
Does it have to be USB 2.0 or 3.0, and what format does it need fat32 or NTFS