Could 3axis cnc router server as 3d?

Sorry for silly questions; with a 3-axis router what is the maximum that you could build for the z-axis. I would've thought you could go pretty deep.
The reason I ask is that I would've thought you could almost get away creating 3d things on a 3-axis by machining one face, then turning it 180˚ and continuing with a seperate file - if you check out here most of it is done flat YouTube - Sculpting a Womans Face [nomedia="http://www.youtube.com/watch?v=euo8IKf2CuQ&feature=mfu_in_order&playnext= 1&videos=jYGc-nHAWpU"]YouTube - Sculpting a Womans Face[/nomedia].
Another basic question: with this video machining a pumpkin YouTube - [nomedia="http://www.youtube.com/watch?v=SuJZnvet6SM"]YouTube - Carving Pumpkins[/nomedia]Carving Pumpkins how does the machine know the size of the pumpking etc, as it's different measurements everywhere. Does the machine scan it first then adapt it's file to suit the pumpking layout?

the pumkin in that video is not real,its a decoration which are all the same and its being cut by a 5 or 6 axis robot router,im sure they have the 3d cad file already for it since they make them there