Hello ariknorr,
Dual-depth-sensor configuration is optimal in your conditions. You'll be able to use both 90- and 180- degrees configurations. Expected capture area is about 2 by 2 meters.
You can also try to use 4 Sony PS Eye cameras, mounting all of them in the room corners at maximum height (near ceiling). In this case expected capture area is approximately the same, but this system is harder in setup, calibration and operation in comparison with dual-depth-sensor configuration. However foot tracking, head tracking and FPS are better in multiple PS Eye configuration.
P.S. Information about minimum required space can be found
here ("Minimum Required Space" row in the 3d table).