I'm drawing this primarily from R. Stark's The Triumph of Christianity chapter 9. The whole chapter tackles the issue of 'Assessing Christian Growth' and discusses some of the issues in creating statistical models of growth in the Roman Empire.

Starks approach is that he takes Acts 1:15, which puts the earliest group of believers at about 120, as a reasonable starting point. He dismisses the claim of Acts 2:41 of around 3000 being added in one day, as exagerration. Instead, he suggest that by 40AD there were perhaps 1000 in the whole Empire. He cites Robert Wilken to suggest that by 150 AD there were perhaps less than 50,000 across the Empire. This, in part, is suggested by the lack of any church buildings until much later. R.L. Fox puts the number at 2%, just over a million, in the year 250. Most agree that by 350 AD, Christians were a slim majority, and so perhaps 30million or so out of 60 million. So Stark takes all this and gives a growth rate of about 3.4 percent, which is comparable for some faster growing contemporary religious groups.