If you mean that you want to precisely measure the length of a very short selection, then this code may be run in the “Nyquist Prompt” (in the “Tools” menu) to display the length of the selection in milliseconds with three additional decimal places:
(setf *float-format* "%.3f")
(format nil "~a ms" (* 1000 (/ len *sound-srate*)))
By the way, I’m just an ordinary user of Audacity these days. I stepped down from the development team when Muse Group took over.