According to Joel Spolsky,
I will give the Internet Explorer team credit. With IE versions 3.0 and 4.0 they probably created software about ten times faster than the industry norm. This had nothing to do with the Internet and everything to do with the fact that they had a fantastic, war-hardened team that benefited from 15 years of collective experience creating commercial software at Microsoft.
This seems like a good benchmark for how big a team it took to create important desktop software in the heyday of desktop software, if you have all the other variables right, if you are maxing out efficiency.
So: how big was the Microsoft team that created the original Internet Explorer?