Hi,
  My coworker Jordi wrote up a useful VMD script for displaying a 
"ruler" grid that automatically rescales along with the molecule.
This script could be modified slightly to draw various other scale bars
if you prefer something different, but it should be a good starting
point as it handles the trickiest parts such as automatically updating
when the user zooms in and out etc.  I've attached the script to this email.

Cheers,
  John Stone
  vmd(-at-)ks.uiuc.edu