Size doesn't really matter that much until he starts adding the Video Recordings which can use a lot of space. If he's a budding Spielberg, go for the biggest you can afford.
Those that are not Mains Powered have a horrible habit of requiring more power than the PC has available, through the USB circuit, when other USB equipment is in use at the same time. They work OK but I think that Mains Power has the edge.
You may wish to discover whether the EHDD has a Fire Wire, as well as USB, connection as this is frequently used by Video Cameras.