Need help with ventilation

Hello everyone, great forum you have here...I'm hoping someone can help me out with my question, I'd really appreciate it!

My friend is trying to figure out how much CFM fan is needed for a 5 x 5 x 7.5 (Total 187.5 cubic feet) room.

A thermostat does not control the fans. The exhaust fan is running 24/7, and the intake fan runs when the light is on only. He read that a full air exchange under 5 minutes is optimal, but that applies to fan systems that are being controlled by thermostats (fans that go on and off according to temperature) he isn’t sure if the same rules apply for fans that run continuously. How do I figure out the right amount of CFM fan needed for his room?

Also, how will the speed of air exachange affect the co2 that's being released in to the air....He is concerned that the extra co2 he's putting in the room will be sucked out before it has a chance to do it's magic on the plants. Any advice is much appreciated.!!

when using co2, turn off the exhaust, keep the intake going....

hope your light is air cooled to keep temps down

Thanks Bud, yes the lights are air cooled. I gues I'll be needing a timer to connect the co2 and exhust fan.

My friend owns a "Producer" by BC Northern lights and the co2 comes on when both fans are flowing. The exhust fans run even when the lights turn off...and the box works great. My friend built a hydro hut and was thinking that he could mimic the Producer's system since it works well for him.