Lin, what you say is partly true. Where the subject falls within (or outside for that matter) the DoF has nothing to do with DoF. But in practicle terms, most photographers (not even me) set the focus distcance and f/stop and then look for an "in focus" subject. Most photographers select a subject, compose, focus, decide what the DoF should be, set the f/stop, shutter speed and shoot. So, for practicle considerations, size matters. Large subjects will require a wide lens and/or a long focus distance (if the photographer wishes the subject to be within the frame and focused). You simply will not get a full body portrait of a 6' tall person with a 500mm lens on a 35mm camera from 10'. NowI suppose you might argue that the only propblem is teh model is standing in the wrong place, but not practicle. True, if that is all the camera I had, I would ask the model to walk down the block.
DoF is not determined by sensor size - directly. Look at the equations for DoF and point to the term for sensor size. Not there. Now you could say that if I selected a 20x24 camera instead of a 35mm for the above example, I might get what I wanted. But that isn't a practicle method either. I usually won't set the model and tripod up and then decide which camera to use.
Another question here. If I set up a shot with an X-Pan camera (focus distance, f/stop, lens) and then switch from regular to panorama mode, does the DoF change? If you say yes, you would of course be wrong. I could then have the film processed, cut the frame in half and change the DoF back again.