From Lexicanum

A Diocese is a region of the galaxy as divided by the Adeptus Ministorum. Each is headed by a Cardinal who might control hundreds of Preachers within the Diocese. Each diocese originally encompassed hundreds of worlds.^1,p5 Ecclesiarch Sebastian Thor, as part of his reforms, reduced the size of a diocese and therefore limited the individual power of each Cardinal, making it harder for any one Cardinal to gain too much power.^1,p16 In the modern Imperium, a single Diocese generally encompasses an entire world.

Sources

• ¹:Codex: Sisters of Battle (2nd Edition)
• ²:Realm of Chaos: The Lost and the Damned, The Imperial Cult