I feel like perhaps this was meant to be a search that accidentally got placed into a new post? Eh well. Just like adults, pediatric patients will obviously vary in size from age to age depending on the particular patient. A quick bit of searching got me a nice youtube video on sizing a NPA for a pedi patient.
"Pediatric Nasopharyngeal Airway sizing" -
Also found a somewhat helpful general sizing chart for AGE with several airway devices...