VDP Sizing depends on many factors VM Storage Size, deduplication, Change rate, Amount of data, retention period etc.
For initial few backups, storage quickly fills because every client that you back up contains unique data. VDP deduplication is more effective when other similar clients have been backed up, or the same clients have been backed up at least once.
After the initial backup, the VDP appliance backs up less unique data during subsequent backups. When the initial backups—the local backups and the backups from other servers exceed the retention periods, you can measure the ability of the system to store about as much new data as it frees each day.
For Example VM With 40GB used storage(Thin provisioned) occupied around 15 GB of VDP deduplicated backup storage first time. After 15 days of daily backup with default retention policy and minimal change rate, total backup storage was about 20 GB.
https://pubs.vmware.com/vsphere-60/topic/com.vmware.ICbase/PDF/vmware-data-protection-administration... Page 27 and 28 gives Sample recommendations for vSphere Data Protection sizing.
