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.