What happens when a country's too small for an airport?
Ultimate way to maintain air service would be to operate helicopters like Vatican and Monaco.
But, international transportation from smallest countries don't have to stick to air. Rail or bus service would be cheaper than air.
In some places even with wide area enough to build airport, it depends on grand transportation.
For example, Kaohsiung, the second largest city more than 300km from Taipei lost almost all the domestic flights except for flights to small islands. That's caused by inauguration of high speed rails between Taipei and Kaohsiung.