Use at least a 12 point font Usability

Choose a font that’s at least 12 points (16 CSS pixels) for body content. If you’re creating content for older adults or people with vision problems, consider using an even larger font size — 14 points (19 CSS pixels).

Research over many decades shows that fonts smaller than 12 points reduce reading speed.

References

• Use a readable font that’s at least 16 pixels - Health.gov
• Font size on the web - PennState
• Statistical profiles of highly-rated web site interfaces. Ivory, M.Y. & Hearst, M.A. (2002). Proceedings of CHI 2002, 367-374.
• The effects of font type and size on the legibility and reading time of online text by older adults - Bernard, M., Liao, C.H., & Mills, M. (2001b). Proceedings of CHI 2001, 175-176
• Readability of fonts in the windows environment - Tullis, T.S., Boynton, J.L., & Hersh, H. (1995). Proceedings of CHI’95, 127-128
• Legibility of print - Tinker, M.A. (1963). Ames: Iowa State University Press.