It is difficult for us to say as it will depend on various things like:
- Array type used
- VAAI capable?
- RAID type
- RTO (for backup)
- Performance requirements
- etc
Maybe these will help:
http://www.yellow-bricks.com/2011/07/29/vmfs-5-lun-sizing/
http://www.yellow-bricks.com/2009/12/23/iops/
But those are mainly about performance, make sure to keep RTO in to account. How long will it take you for instance to restore a 2TB LUN?