I'm not sure if you've found much by way of google but this search bought up this page as hit 1: http://www.oreillynet.com/xml/blog/2006/03/more_statistics_on_user_clicks.html
Incidentally this question is on page one of the search too :)
Its from 2006, but the first bit has a little about the font size settings in IE.
I doubt there is any documented data for this specific thing.
One thing I can tell you is that in over 10 years of working around computers I've seen about 1 in 10 users having a above default DPI setting. eg. 120 that caused the font size to be bigger than 16px in their browser. It also caused layouts that weren't styled properly to break.