I learned the hard way by getting a crappy 4" fan to start with, I have a smaller space than you and it got stupidly hot even with a 200w CFL when it was warm outside. Like you I couldn't decide what to get so I went for the biggest feasible. I've ended up getting a 6" rhino thermostatic fan off fleebay cheap which is doing a great job with a 400w hps. Getting your ventilation right is one of the most important things I have learned.
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