with a 400 watt light i would recomend 3x3 to 4x4. remeber that in a open area light will be wasted so haveing walls close with good with paint will be great as for plant numbers its depends indica or sativa, your preferece on size and your method of growing sea of green or scrog. you can get alot of little indice plants in that area 1 per foot no problem or as few as on big sativa. the bigger the pot the better but you need to decide what your growing and how big first. share with us your plans so far! good luck