[RangerDanger]

Determine the amt of floor space by the effective range of your lights.
A good rule of thumb is 50 watts HPS or MH per sq/ft.
Therefore, a 400-watt light effectively covers 8 sq/ft, or slightly less than 3' x 3'.

Ventilation:
Marijuana, like all living things, need fresh air.
Marijuana, like all plants, "inhale" CO2 and "exhale" oxygen. When all the available CO2 is used up, growth ceases.
That means intake and exhaust fans.
Ideally, there should be a complete change of air every minute. Figure out the cubic feet of your grow space and get fans that will accomplish this. Fans are rated at cfm (cubic feet per minute).
I've found that for a small space bathroom exhaust fans work good, for a larger space such as you have I'd go with squirrel-cage fans.

Carbon filters and negative ion generators work well to eliminate odor. Some people put bowls of vinegar in their grow rooms to absorb odor.