I have not yet bought a 6 TB drive. I spotted this reply over in the Drobo Forums:
so I checked the Capacity Calculator and the largest drive listed is still 4 TB. I'm still at 19% free (was 18% last month) so I'm just going to hold off on importing any addition DVD and Blu-ray content for now.
...currently 6TB drives are being tested and qualified for use with our products. The 6 TB drives will not be added to the calculator until they have been fully tested and qualified for all the pertinent units, I apologize for the delay in adding these, but we do need to complete the qualification process prior to adding them to the calculator.