400 HID is perfect for a 9 square foot room. That is 5,000 lumens per square foot for MH(Veg) and over 6,000 lumens per square foot for HPS(flower). That is more then enough light for that room and you will be able to control heat and get lights closer then with a 600. For any room larger then 9, say once you get to 12 square feet and above, use 600s.