The question was “How many users browse the main Internet Archive site with a default font size other than the common value of 16 pixels?” By knowing this, we would determine how many users would be affected by sizing with relative units like rems/ems.
Using the methodology I describe below, we found that the answer is 3.08% of our users.
So if you set type in pixels, and your traffic is anything like the Internet Archive's, 3% of your users won't have their explicitly-asked-for font-size alteration accommodated.
As ever, there is more to think about. How does page zooming factor in? How annoyed are those 3%? How annoyed are the 3% on other sites that use pixels but set type pretty large anyway? How much do we care about people who use the preference to size type down?