Size of Time Machine backup hard drive
How large a backup external hard drive do I need for a 1Tb iMac?
OS X El Capitan (10.11)
A minimum of 2-3 times the capacity of the source disk. For your case that would be 2-3 TBs. To avoid eventually running out of space on the backup drive bigger will be better. Time Machine will fill up the backup drive over time because it makes additive backups.