I have some numbers on the needs of any vent system and for my system specifically.
For a passive ventilation system, the vents need to be 20% of the size of the square footage of the growing space. so a 200 sq ft space would need 40 sq ft of ventilation (eg. wide open windows or doors).
A mechanized ventilation system, with fresh air intake and exhaust fans, requires much smaller vents. The exhaust needs are calculated in Cubic Feet per Minute (CFM). Do this by basically calculating your air volume. This is roughly Length X Width X Height. But for a hoophouse, because of the bent hoops it's a little harder to calculate. You need to calculate the total area of the end wall, then multiply by the length. In my case, my air volume is 1042 cubic feet. Professional greenhouse equipment suppliers recommend a ventilation system with a CFM measure at 1.5 times the air volume, so my fan needs to be 1,563 CFM. The ventilates the entire space faster than once per minute.
Also, in terms of efficiencies, the fresh air intake vents should be positioned low on the greenhouse and the exhaust fan should be positioned up high. This makes sure the coolest air possible is being pulled into the warmest part of the greenhouse--the top. The exhaust fan should also be positioned downwind so it is not fighting to push air out but can flow easily.