These are the standards set by the National Library For The Blind And Physically Handicapped Materials Development Center.
Dot height: .020 inches
Dot spacing .09 inches
Character spacing: .240 inches
Line height: .40 inches
Interpretation: From dot 1 of the first braille cell to dot 1 of the next is .24 inches. Therefore, one inch of line length yields approximately 4 characters.