Aerodromes can specify the largest aircraft they can accept; often wingspan is a critical aspect.
Another aspect is based on length & width of aircraft for fire categories 1-10. (UK CAP168 gives details) where a licensed airport is a requirement.
In practical terms, the operator would conduct the airport authority if uncertainty existed. Not every airport could accept an Airbus 380 based on either the airfield’s physical size including PCN/ACN or equipment etc.
En route designated airports will be nominated as ‘bolt holes’ should a diversion be necessary especially on ETOPS flights.
As you pass one equal time point between these airfields, the next one is the bolt hole.
Think about the practical aspects as oppose the academics…