Creating a large motion sensing plane
It looks, in principle, like a pair of quite cheap video cameras will do the job. With fairly wide-angle lenses to give you 90 degree field of view, you'll also get close enough focusing that you can cover close enough to the cameras to meet your 1 foot requirement. Obviously, you can get USB video cameras to send the data to your PC. Analyzing the resulting images is beyond my competence. Certainly you ought to get good enough resolution to identify multiple hand-size objects.

There do exist line cameras which will give you better resolution in a single plane, but these tend to be more expensive than you are willing to spend.