Steve mentioned the razor blade he engraved on was 0.005" which is 0.12mm by my reckoning. Graham suggests his blade was 0.1mm.
Looking at Graham's 15 line engraving of the Lords Prayer, on a 2mm pinhead, with generous line spacing would be (as a guess) less than 1mm in height of each letter. My guess would be .8mm letter height but that is a best guess.
It would of course be great to have some very specific sizes.
Just off topic ever so slightly, How about a letter 5 atoms high?
http://www.sciencemuseum.org.uk/ante...urself/153.asp