Grow room size for 2 400w HPS?

[Weed4Life]

I need to make the hoods for these lights but what size of room can I build and how many plants can I have under them? I was thing 5x4x5.

[4x5]

that would work fine, heat will be a concern, spend the dough and get good ventilation.

[Weed4Life]

I was planning on buying a squirrel fan for exhaust and maybe 2-3 fans on bottom for intake and a fan inside circulating the air. How many plants can I grow in that size of box with those light if I was doing soil?