1.3.1-4 Character Size for Text Readability

The height of characters in displayed text or labels should be at least 16 minutes of arc and the maximum character height should be 24 minutes of arc.

Additional Information:
Character heights of 20 to 22 minutes of arc are preferred for reading tasks. Slightly smaller characters are acceptable in high-contrast panel labels (see Section 11.3.1.5, Label Lettering). Characters should not be larger than 45 minutes of arc when groups of characters are displayed. Minutes of arc can be converted into height as follows:
    Height = 6.283D(MA)/21600
where MA is minutes of arc, and D is the distance from the user to the screen.