Sniper New Jersey License Plate? NDA 21Z

NDA.... shortest word NaDA as in "nothing", which is what the sniper has proven with his cowardly actions

longest word? couNterDemonstrAtions.... 21 letters (hmmm same as the number contained in the plate)

Adding the "Z" into the equation: NDAZ
shortest? vaNDAliZe (9 letters)
longest? iNDustriAliZations (18 letters)