You need to determine the amount of air that the cooker hood needs to extract all the fumes, steam and odours.
i.e. cubic metres per hour = m3/h.
Establish the measurement of the kitchn length x width x height.
e.g. 5 metres long (L) x 4 metres wide (W) x 2.5 metres high (H) = 50 cubic metres.
One needs at least 10 changes of air per hour.
50 cubic metres x 10 = 500 cubic metres per hour.