We recommend a minimum of 4pt text at 100%, black on a white background. Fonts vary on how clear they look when they are printed out, but in general, 4pt is about the limit. It’s not just the technical restrictions of the print process, people would find it pretty difficult to read anything that small even with perfect vision.
If you have small white text inside a dense black colour we advise your text to be no smaller than 6pt and as bold as possible due to the natural ‘filling in’ of ink. If you have less of a colour contrast a larger, more prominent font style and size might be required. Try printing your design on a standard printer first to see what it looks like in the flesh as computer monitors can be deceptive.