Amount of sq. Ft for a 400w hps

I have a 400w hps and in the past i wasnt getting as big of harvests as i should have been getting, only 3 oz from4-6 decent sized, single cola plants. I think i had them in too big of a space and the light was too spread out. Does anybody know the proper sq. ft for this size light?

About 8 sq. ft. (the "squarer" or "rounder" the better). For example, a 2'x4' box will have a much longer light path to reach the far end of the box.
A little less than 8 sq. ft. will give you more light per Sq. ft. Much more than 9 sq. ft. is starting to "spread it thin". Some may disagree, but I say a 36" diameter "cylindrical box", with the plants turning slowly on a motorized "lazy susan" is "ideal"!