If there is extensive research on the performance of very small teams, he did not present it. All that he presented was one graph from Lawrence Putnam for schedule and effort to complete a medium sized project for various team sizes. Medium was defined as 35,000-95,000 lines of code, and averaged 57,000 lines. (The averages were close to this for all team sizes.)

Eyeballing the graph, it looks like teams of 1.5-3 people completed their projects in about 14 months, 3-5 in about a year or so, and 5-7 in just under a year. No data is presented about the relative quality of those projects, or the distribution of languages used.

If you wish more detail than that, the reference that he gives is Putnam and Myers, 2003. Which would be Five Core Metrics. I have not read that book.