There's definitely some truth to the concept that the bigger it gets the less likely it is to remain unified.
However, I think one of the biggest factors that could allow growth/spread without driving dissent/division would be to basically have a somewhat "bigger tent" of beliefs. Certain beliefs could be codified, but they could avoid some degrees of specificity that made some view heretical though they weren't all that different. For example, the Nicean Creed codifies the view of the Trinity and rejects anything different as heretical. Yet, the distinction of the interpretation of the Trinity has always seemed somewhat arbitrary and unnecessary. If two people believe the same on 95% of doctrine, but one believes that the Godhead is 3 personages in some kind of amorphous grouping, while the other believes that the Godhead is 3 physically separate beings, is THAT distinction really the important thing? Yet, when you codify things and state that anything else is wrong, then you inherently drive a wedge.
You could still have some "standardization" without that degree of specificity. You could have a pope, or patriarchs, agree to a broad set of beliefs that are mandatory to be considered "legal," but local churches could vary in other things.
Other than that, a much earlier defeat of Islam would have increased the coverage of Christianity considerably (Middle East, N Africa, Central Asia) and left Christianity better positioned to continue expanding in Asia.
I am not sure if it is possible keep Christianity united. There was already many Christian nominations during early centuries. You might get world where Christianity is wide spreaded than in OTL but probably not so unified. Or you can get unified Christianity, but it probably should be quiet small and probably united by common enemy.