There doesn't seem to be a formal definition of what constitutes a microstate.
I propose using the Wikipedia definition:
'A microstate is a sovereign state having a very small population or very small land area, but usually both. The connection is restricted to sovereign states with an area less than 1000 km² AND fewer than a million people.'

There are 24 entities in the world that fit this description (see microstates). Cyprus, Lebanon etc. are therefore not admitted. They belong more to the Belgium-Israel league.

If anybody knows of a better one, please let us know!

Some Possible Sites for this connection; using the wikipedia definition

Madriu-Perafita-Claror Valley (Andorra)
Morne Trois Pitons National Park (Dominica)
Historic Centre of Rome, the Properties of the Holy See in that City Enjoying Extraterritorial Rights and San Paolo Fuori le Mura
(Vatican City)
Vatican City (Vatican City)
Brimstone Hill Fortress National Park (Saint Kitts and Nevis)
Pitons Management Area (Saint Lucia)
San Marino Historic Centre and Mount Titano (San Marino)
Aldabra Atoll (Sechelles)

Possibly Qal'at al-Bahrain – Ancient Harbour and Capital of Dilmun -bahrain has less then 1000 km2, but just over a million people

Sorry I Forgot a few sites

Vallée de Mai Nature Reserve (Sechelles)
Chief Roi Mata's Domain (Vanuatu)

Hi Xeres,

Most of them are connected now. You're right pointing out Bahrain, that isn't a microstate anymore. Vanuatu also isn't, it's bigger than Cyprus or Lebanon.