People do successful grows inside of PC cases. So you're talking cases that are as small as 16" high by 16" deep by 6" wide.
That's probably around a practical minimum, I think. Its certainly "possible" to go smaller, but at some point you're doing all the work (or more) of a full-sized grow only to yield a dime-bag's worth of buds!
Key to these "microgrows" is to use every trick to keep the plants short:
-Pick short strain (lowryder is good).
-Keep plant close to lights to minimize stretching. Good ventilation is a must!
-Use short pot to minimize vertical height.
-Small volume pot helps keep overall plant size down.
-Train plants to convert vertical height to horizontal spread . . .(Bend sideways, use screen/scrog, etc).