For air exchange purposes (to maintain co2) use the whole space if its free moving air (not a tent in a space) You need the volume not the area so need to include the height too.
For temp control, heat removal purposes use the following calculation to find the approx. cfm required to stay at the desired air temp.
It boils down to CFM = 3.16 x Watts / DT(°F)
Watts is the lighting power (and any other 'hot' things in the grow space) in Watts.
DT is the allowable temperature rise within the enclosure (i.e. desired temperature minus ambient temperature) in °F.
So, to work out the size of fan required (in CFM) simply plug your lighting Wattage into the equation along with the DT value.
Personally I would go with the second calculation