https://www.mongodb.com/docs/manual/core/capped-collections/#create-a-capped-collection is out of date.
Needs rework:
"If the size field is less than or equal to 4096, then the collection will have a cap of 4096 bytes. Otherwise, MongoDB will raise the provided size to make it an integer multiple of 256."

Description of Linked Ticket

SERVER-67246 unlocked the ability for the oplog capped size to be anything, not just a multiple of 256. This blocks fresh installs from downgrading.
I think we should fix this by adjusting the oplog cap size algorithm to go back to rounding to the nearest 256. This will avoid problems with on-prem LTS users on 7.0 downgrading to 6.0.