Hello,
The Virusdef folder for Symantec Endpoint v12.1 would be under following Locations: -
Win XP - C:\Documents and Settings\All Users\Application Data\Symantec\Symantec Endpoint Protection\CurrentVersion\Data\Definitions\VirusDefs
Win 7 - C:\ProgramData\Symantec\Symantec Endpoint Protection\CurrentVersion\Data\Definitions\VirusDefs
Server 2003 - C:\Documents and Settings\All Users\Application Data\Symantec\Symantec Endpoint Protection\CurrentVersion\Data\Definitions\VirusDefs
Server 2008/R2 - C:\ProgramData\Symantec\Symantec Endpoint Protection\CurrentVersion\Data\Definitions\VirusDefs
Again, in reference to the Question "approx. size for the Symantec Endpoint v12.1 Virus Defs folder", check this Article:
Drive Space used by Virus Definitions Updates
http://www.symantec.com/docs/TECH141811
Disk Space Management procedures for the Symantec Endpoint Protection Manager
http://www.symantec.com/docs/TECH96214
Hope that helps!!