Phil:
Depends on your breed of birds. Larger heritage breeds should have about 12" x 12" (which is the size of our milk crates). I made mine a bit larger at 14" x 14", plus a sloped roof on top, angled at least 30 degrees so they cannot perch or roost on top of them.
You might get away with those smaller boxes with bantams and smaller breeds like leghorns.