It depends on what you mean by "retreat". It can mean a withdrawal, as in a military retreat. It can mean a time of quiet and solitude. It can mean a place for quiet and solitude, like a refuge or sanctuary.
Paganism is the worlds largest retreat either meaning decline, quiet, or solitude.
Roman Catholic
The world's largest WAN is the internet.
The largest desert in Asia is the Arabian Desert and the second largest is the Gobi Desert of China and Mongolia.I'm sorry, but the Taklamakan Desert ranks as the 16th largest in the world and 6th largest in Asia.
Ture, the Canada does has the world's largest bay.
largest volcano in the world is Mauna Loa in Hawaii.
Could well be Ashram, a hindou religious retreat.
Vedas
A Hindu Retreat would be any place where an individual could meditate and grow closer to the Hindu religion. It would most likely be in a natural environment where there is peace and harmony and people of all faiths would be welcome
Ashram
Revival
Monastery
The largest religious monument in the world is Angkor Wat in Cambodia, which is a Hindu temple complex originally built in the 12th century.
The largest religious group in the world are Christians. There are over 2,200,000,000 Christians worldwide and the religion is still growing strong. This is almost 1,000,000,000 more than the second largest religion, Islam.
Islam - the second largest religion in the world
Angkor Wat
Ashram
Muslim Hajj, every year, 3.5 milion