Almost any light over 250w in that small room will make it too hot. I would get a cooling hood so the light can be cooled seperatly.
But you can try a 400W with some fans, but it would be hot and if it gets hot in your area you'll have problems. Just not a 600W without some powerful cooling, put a 500 watt heater in the closet and see how hot it gets, thats what a 600W will do.