I think you're mistaking "common wisdom" for research. A lot of people like to say 7 +/- 2, but as I understand it, that number comes from studies of short-term memory. (It's why phone numbers in the US are 7 digits.) Research of teams (not specific to Agile) puts the maximum effective size closer to 12, and the maximum size of a "tribe" around 150-200.
My own experience is that 12 people works great for an agile team, with 6-8 of them being programmers. If you're not using pair programming, cut those numbers in half.
Yeah you are right, I asked Larry Maccherone to see if there was prior research about that common belief in the 7 person team and he was not aware of any.
If you are interested in hearing about some of the current research that we are doing about team size you can check into it on this video. I am not sure he is ready to publish his work but this can give you a taste of it.
My own experience is that 12 people works great for an agile team, with 6-8 of them being programmers. If you're not using pair programming, cut those numbers in half.