250w would be the smallest you can go in a space that size. Your harvests won't maximize the efficiency of the space without supplemental lighting, but you can still get top quality bud.
400w would penetrate the 30-36" inches of usable growing height much better, which means bigger top buds and a lot better lower buds.
I'm with the rest, if you can provide an excess of exhaust and use a glass heat shield of some sort I'd go with 400. Otherwise, a 250 will get the job done.
Thats how im planning on doing my cab, but im thinkin about using a 600 watter instead with 2 fans doing push/pull. It also allows you to seal off the light from the room itself and will allow for better co2 management.
I'm not sure i dont see where it would matter. Technically the one pushing will lose velocity before it reaches the second fan. So i would imagine that the air coming from the one pushing would be slower because of the resistance in the reflector.
So you're saying that one fan would do be able to do the same amount of cooling if you don't have one pushing and only one pulling? I mean it just seems that if you had one before blowing cooler air on the bulb it would cool the bulb then push the heat out and another would help pull that heat out...