About an inch. atto- is the standard SI prefix for multiplication by 10^-18. A parsec (parallax-second) is 3.26 light-years; an attoparsec is thus 3.26 Ũ 10^-18 light years, or about 3.1 cm (thus, 1 attoparsec/microfortnight equals about 1 inch/sec). This unit is reported to be in use (though probably not very seriously) among hackers in the U.K. See micro-.