From the makers’ manuals
How big should a sauna vent be?
Under the UL 875 rule quoted in the Finnleo and Amerec manuals: rooms under 31 ft² need at least 9.3 in²; bigger rooms need 0.3 in² per ft² of floor, with the outlet about 2 ft above the inlet [Finnleo Himalaya manual]. Harvia specifies a 50–100 mm supply and an exhaust about twice that, 100–200 mm [Harvia Spirit manual].
How many air changes does a sauna need?
About 6 air changes an hour (Harvia, Saunum). EOS asks for at least 5 room volumes an hour [Harvia Spirit manual].
Where do the sauna vents go?
With natural ventilation, Harvia puts the fresh-air supply below or beside the heater; with a fan, above it. The exhaust sits low and far from the heater [Harvia Spirit manual]. For mechanical ventilation HUUM puts the supply at least 400 mm and the exhaust at most 600 mm off the floor [HUUM Drop US manual].