PRN - Praslin in the Seychelles would be up there.

Other than that, PUY Pula in Croatia would also count as small in my book.
Other than that, Proserpine / Airlie Beach, Australia (PPP), Samui, Thailand (USM) are also pretty small.

Now, undersized airports are much less pleasant. Thinking KGS Kos, Greece and REU Reus, Spain as too small for the number of passengers they are handling.

Mark