with adequate ventilation you could use a 600w HPS in there. you should be able to get 6-8 in there comfortably, 10 if you want to stretch it.
you'll need a pretty hefty fan to ventilate that room with a 600w HPS - something in the range for 400+ CFM.
the room is 6x4x7 feet cubed = 168. You want to change the air at LEAST twice a minute, that's 2x168 = 336CFM. If you're going to use a carbon filter then add another 20% - thats over 403.2CFM..Somewhere around the 500+ mark would be much better and give some headroom for summer.