The space required by a dSource or VDB will depend upon your database size and usage. As a result, the Delphix Engine cannot give any exact numbers as to how much space will be needed.
However, you may be able to determine future space requirements by analyzing current usage. You can do so from the Capacity Management screen. To access this, log in to your Delphix Engine and navigate to Resources -> Capacity. You can see the space usage for all of your databases here.
I hope this helps. Please let me know if you have any questions or concerns and I would be happy to help you out.