This is how I have given levels to the airports + a example:
1 - dirt tracks (Celje Airport)
2 - less then 1000 metres asphalt runway (Lošinj Airport)
3 - over 1000 metres asphalt runway (Bol Airport)
4 - less then 3000 metres runway (Maribor Airport)
5 - over 3000 metres runway (Dubrovnik Airport)
6 - over 3000 metres runway, not big difference between level 5-6 (Ljubljana Jože Pucnik Airport)
7 - ? None have got level 7, yet.
8 - needs to be a big military airport (Slatina Air Base)
9 - ? Zeljava maybe
10 - I guess Belgrad, Nikola Tesla airport can only match this