There's mild controversy about "Cosmic Report Card: Earth" because the text can't be understood without the title- so is it really a one-letter story?
Another one-character story is "If Eve Had Failed to Conceive" by Ed Wellen (Orbit 15, ed. Damon Knight)
"."
(As "." is smaller than "F" - maybe Wellen beats Ackerman.)
Incidentally, "Cosmic Report Card: Earth" can be downloaded from
http://www.lulu.com/4forry for £3.44 (print version £6.47)

- does that make it the most expensive SF story ever sold in terms of price per letter?