Hey man,

I have 2 plants growing in a 2x4x7 and it only holds 2 plants and there is not room for anymore. I have a 250 watt HPS in there and it has worked great so far. I also have the 4 inch inline fan, which I bought from HTG. Hope that helps a bit. I would get the 4x4x6.5, because if you get the one that is only 5 foot tall then it might be hard to keep the light far enough from your plants if they get fairly tall.