To determine the size of an airport, we use the length of the longest runway.
|Max Runway Length||Airport Size|
|> 10 000||5|
|> 8 500||4|
|> 7 000||3|
|> 5 000||2|
|> 2 300||1|
For all the little airfields coming from MSFS which were not currently in our database, we don't have the runway data and we automatically set the size to 1.