Complex sentence: One of the great centres of Orthodox monasticism is Mount Athos in Greece, an isolated, self-governing peninsula approximately long and wide (similar to the Vatican, being a separate government), administered by the heads of the 20 monasteries.
Split into simpler sentences: One of the great centres of Orthodox monasticism is Mount Athos in Greece, which, like the Vatican State, is self-governing. It is located on an isolated peninsula approximately long and wide, and is administered by the heads of the 20 monasteries.