The main concern related to font size is that text can easily be made too small for many users to read.
12 Point for Body Text
For most documents, body text should be around 12 points. Fonts at smaller sizes may be illegible for some audiences.
Nine Point Minimum for Footnotes
If a document contains footnotes or endnotes, the minimum size should be about 9 points.
If a PowerPoint file is meant to be projected before a live audience (e.g. during a class lecture), the minimum font size should be about 24 points. At smaller sizes, the text becomes illegible on a screen.
Headers and subheaders can be much larger, between 40-60 points depending on the slide layout.
Font Size and CSS
See the Font Size on the Web page for information about CSS best practices.