64cm = 21 inches, 50cm = 19 inches, 45cm = 17 inches .. so 21 inches high 19 inches wide and 17 inches deep..
I've never ran a 400watt, but I do run 2 600's and without the fans blowing on them they run so hot you can't touch the metal housing. However, only about 6-8 inches away its warm and acceptable. So the heat really fades quick, I run an open reflector with an $8 walmart fan blowing on it and that was enough to cool it to the point that I could hold my hand onto it...
So not sure about the 400watt, but a 600watt would never work in there... After you account for your growing pot, room to grow, and some airflow... You'd have nothing left over but some branches and a light. I recommend CFL. A nice Y connector on the top of the box with an extension cord you could run 4 CFLS, get the 43watt blubs at about $10 a piece.