I have a 3' x 3' under a 600 watt right now. You are on the right track. Just watch and make sure your fan is pulling enough. I am running a 6" inline axial that is rated at 480 cfm, but with tubing and the light in the way it pulls less.
Give it a go, but be on the watch out for heat. I can touch the top of my fixture all day long, but it still kicks some heat out the front .
Stick with the 600 watt... It will give you the best results with the least heat, and you can always add a second one later if you want to upsize.