Quote Originally Posted by keylime
Well, that all depends on how large they are when flowering, so there is no ONE correct answer. 400W is a good size for the space. 400 is not as penetrating as say a 1000W, so I would not have them too large. If it was me, I would start the flowering cycle when they reach 8" or 10". That means when they finish they will be approximately 2ft high. If you had them that size, and say your plants are in a pot that is 10" high, and you want to stay 1 ft above the tops of your plant and the light....and the light needs 6" under the ceiling, you would need 10"(pot)+24"(plant)+12"{top of plant to bottom of light)+6"(light), your box should be at least 4 to 5 ft high.
As for the number of plants, I would be shooting at around 20. As I said there is no ONE correct answer.

keylime
Thanks for the reply, I wanted to know so I could build the box around a hydro system that had the amount of pots that would fit my box. But I'll start out with a simple soil system while I work out heat management and what not.

I was planning on making the box 6' tall, but I can go as high as 8 if that makes sense.