I recently finished a project that required a fair bit of research. The data I gathered was in the form of survey responses which indicated what color combinations the respondents found readable, and what they did not. The survey was in the form of a small computer program which allowed me to collect a lot of data, quickly.
My key focus was on the issue of text colors over a series of randomly generated background colors, but at the start of the survey, the respondent was requested to adjust the size of the font to that which was most comfortable for his reading.
The result of that request? About 90% of the respondents chose to set the font size to somewhere in the 14 to 16 point range. This was for a starting screen where the colors were not an issue (black on white) and the font was a conventional serif font.
Since most people use a somewhat smaller font size in their site designs, I thought the info might be useful.