There's no way of stating this definetely.
Some people have a hard time seeing small fonts, like this on the other hand some people find fonts like this hard to read. This is based strictly upon the size of the font.

Most people find a font size of 11 points easy to read from a distance of about 1 1/2 - 2 feet (45 - 60 cm).

Other aspects that need to be taken into consideration, are font style, foreground colouring, and background colouring.