So the answer depends. How many end user computers will have the fog client installed on? How big (computer wise) is your campus? For a small site FOG will run on a Raspberry Pi3, but that doesn’t scale well.
Most systems we would start out with 2 vCPU and 4GB of RAM. For the host OS disk 50GB and then I would recommend creating a second disk just for the images and a 3rd if you plan on having a lot of applications deployed by the fog server. The reason for independent disks for images and snapins is that you can expand a dedicated disk easier than the disk your root filesystem is on that is used by your linux OS. That is a bit deeper of a subject than what does fog run on.