About 31 mm (one 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 * 10^-18 light years. Thus, one attoparsec per microfortnight is about one inch per second.

This unit is reported to be in use (though probably not very seriously) among hackers in the UK.

[jargon file]