Most likely .5TB. This topic is fairly well documented in the admin guide on page 79. http://pubs.vmware.com/vsphere-60/topic/com.vmware.ICbase/PDF/vmware-data-protection-administration-guide-61.pdf
What is not well documented:
- In my experience increase the ram of the .5TB appliance to at least 8GB. 4GB minimum is not enough for most VDP deployments with a data domain.
- I personally have never run into a VDP environment that has needed more then a .5TB appliance when used with a data domain. This is including some with 150+ VMs and over 30TB of data protected (larger then that and I'm probably going to use a different solution anyways)
- My understanding is if you do a lot of transaction log backups you will run out of space on the .5TB appliance as the meta data stored on the VDP appliance is highly dependent on the number of unique files.