Religious Offshoots of Christianity Ranked by Population This seems like the best place to ask this question from the sites:

There seems to be a clear consensus on the religions ranked by population, with Christianity at the top with a little over a billion, then Islam, ect.

Upon looking for data on the top religion where the various offshoots are distinct instead of lumped together as their parent religion.

My question is: is there data on the ranking of Christianity including offshoots by population, and if not is there any reason there isn't other than that people didn't consider doing this? Are there any ballpark estimates about the largest groups within Christianity?

This is a post on this site that has some data, but I'm curious if there's anything more comprehensive
This is the most comprehensive survey (dated December, 2011) that I could find.  You will need to examine this lengthy report yourself for more detailed information but the following extracts may be useful:


  About half of all Christians worldwide are Catholic (50%), while more than a third are Protestant (37%). Orthodox communions comprise 12% of the world’s Christians. Other Christian groups, which make up the remaining 1%, include the Church of Jesus Christ of Latter-day Saints (Mormons), Jehovah’s Witnesses and the Christian Science Church.   Muslims, the second-largest group, make up a little less than a quarter of the world’s population  http://www.pewforum.org/2011/12/19/global-christianity-traditions/


The following link provides information on how Catholics, Protestants, Orthodox and other denominations are categorised: http://www.pewforum.org/2011/12/19/global-christianity-traditions/#defining

Here is a brief statistical summary from the findings within this report:


  Total Christian Population Worldwide:  2.184 billion (31.7% of world population)
  
  Of which:  Catholic    50.1%
          Protestant  36.7%
          Orthodox    11.9%
          Other       1.3%
  
  Within Protestantism:
  305 million Charismatics
  279 million Pentecostals
  285 million Evangelicals
  
  Largest Distinct Protestant Denominational Families:
          Anglican (11%)
          Lutheran (10%)
          Baptist (9%)
          Reformed/Presbyterian (7%)
  
  Total Muslim Population Worldwide: 1.6 billion (23.4% of world population)


Certainly, this report gives "ballpark" estimates about the largest groups within Christianity.  It is possibly the most detailed and comprehensive survey available, even though these statistics are 8 years old.  I hope this helps you.