go as big as you want. if you don't have the proper ventilation you won't be able to control the temps of a 400 or a 600. So it all comes down to your setup, not just your grow area. Temp of air going into the grow room, filter CFM, fan CFM, Cool Tube/Vented Hood, etc... all these factors come into play. if you're not going to use a vented hood or cool tube, you might run into issues with temps (Hot and/or cold). What is your setup?