most international airports are no where near that big, I just had a quick look on google maps at a few airports and the biggest I found was heathrow and that’s only around 2.5kmx5.5km(I also looked at a few US airports but those ones were all smaller then heathrow) so you could make most of them in a 5kmx5km map.
you don’t need to go that small, if you want to use a different scale I would recommend just using 1UU as 2CM that way you double the area to 10kmx10km which is far bigger then any normal airport, in addition going much smaller then about half scale can cause issues with physics…ect so if you want it to be a game I wouldn’t go much smaller.
also the world composition tool is probably your best option if you want to keep the 1UU=1CM scale.
hope that helps:)