The Russian Orthodox Church proved to be the largest in terms of territory IOTL, though there could be chances if the Turkic tribes convert to Christianity of the Byzantine kind way before the Mongol invasion.
The Pope never breaks from the Emperor of the Romans. The longer the Byzantines prosper, the more likely that is. Potentially, there could never be a schism in Nicene Christianity before any successful Reformation movements.
The whole world. Christianity doesn't schism or if it does a healthy Roman Empire quickly puts it down. Slow reconquest of the West. Visigoths fall, Franks are supported as a big vassal and get Britain and Germany under their control. Persians are crushed and the Romans reach the Indus (slowly most likely). Franks unify with the Romans after a succession crisis (like OTL Frank split) and the Emperor steps in to restore order. Islamic raiders are beaten and scattered. Slow conquest, colonization, conversion of eastern Europe. Turks are beaten and subdued around 800-900. Slavs are brought into the fold, Hibrenia(Ireland), and Caladonia(Scotland) as well. Mongols are beaten off. Invasion of India. Colonization and Conquest of the world. A good solid Roman-wank basically.
In my timeline Light in the Darkness(The Legacy of Cyril and Methodius) Saxons and the West Slavs initially side with the Orthodox rather than the Latin Church.
Islam doesnt happen, monophysitism takes its place, churches reunite on a miaphysite basis. Expanded connexions let ethiopa and partners evangelize all of africa.