If you include a few more details and pictures I can probably assist you better, but this should help.
Air should be exchanged every one to 3 minutes +/- depending on you grow size, temp., relative humidity.
GROW SPACE SIZE WATTAGE OF LIGHT CFM OF FAN
2’ x 4’ x 5’ 250W 80-120 CFM
3’ x 3’ x 6’ 400W 108 - 162 CFM
3.5’ x 3.5’ x 6.5’ 600W 160 - 238 CFM
4' x 4' x 7' 1,000W 224 - 336 CFM