the diameter should be equal to the diameter of the pinkie finger and the you measure the distance from the tip of the nose to the earlobe. 28fr is common size for and adult male.
The answer stated above is correct... however, if it's a grid down situation what are you planning to do with this patient next? If there is a situation severe enough this intervention is needed do you have someone in your group qualified to take the next step/steps? Not meant to be picky but...
I agree that some medical items that preppers like to have would only be good when everything is normal and the patient can be transferred to a higher level of care. I have various types of airways to be utilized when necessary but they would not be of any use if that patient could not receive a higher level of care promptly. I am trained and certified in their use along with other items for patient treatment that would only be effective if the hospitals are still operational and excepting patients.
I'm kind-of an airway nut.
I keep adult/child: ambu-bags(with peep valves), NP airways, airway hooks, LMAs, endotrachial,oral rae,nasal rae tubes, along with miller/mac 0-1-2-3 sized blades(disp.handles too) in my airway kit. There are some foreseeable instances when sustained assisted ventilation could be useful even when there is no next level to transport to. Unique circumstances like contact with paralyzing botanicals/stings or accidential CO poisoning(extreme case), etc...
May I add that once you do measure as per the above, if you need to shorten it cut at a 45' angle, then insert in the right side nostril with bevel towards the Nasal Septum rotating is necessary, after lubing the NPA with Surgical lube or if necessary sterile water, PT own blood. Do not force the NPA in.
Contraindication to the use of NPA: Signs and symptoms of Head Trauma.