VMware 5.5 supports a VMFS datastore volumes up to 64 TB in size. The nimble Array can support a volumes size up to 128TB.
So you have to ask yourself, "How big do I want to go?"
I don't see people making datastores much larger then 2 TB except when used specifically for large Databases, or shares. The Nimble snaps are amazing for recovery and clones, so must of the rules around RPO and RTO are removed with a Nimble array. The reasoning behind a 1 TB LUN was that you can restore a 1 TB volume from tape faster then a 2 TB Volume, and a 500-GB volume even faster. But if your going to be recovering from a SNAP or replicated SNAP, then the recovery time the the same no matter the LUN size.
But - if your going to run external backup software - this is were you might want smaller Volume. Because of that time to recover, and backup files.
The best advice I can give is, "what ever your comfortable with for recovery times, and that your backup software can complete within your SLA".