The size of the characters to test 20/20 vision ?
1 minute equals 1/60 of a degree
This is the “normal” minimum discernible resolution for a human eye when tested for character recognition.
D = 2 x 20 feet x Tan (O/2)
Hence the letter E will be 9 mm x 9 mm if intended to be read by
a person with 20/20 vision at a distance of 20 feet or 6 metres.