Here is a specimen from the Discovery Gallery hands on fossil exhibit at the 
Louisville Science Center.  It is labeled as a 'Club Moss Root' but no location or time period included.  I do not know much about fossilized plants but most of the fossils I have seen are from the Mississippian or Pennsylvanian periods (Carboniferous). 
Research at this 
site about club mosses they were known as lycophytes of the genus 
Lycopodium (
lycos Greek for wolf and 
pous Greek for foot).
I did not have a ruler but the specimen seemed to be about 15 cm long and 6 cm in diameter.