Snapshot size can be 10-12% of the total size of the NAS logical volume 48GB or 3 times the amount of changes or new data that will be written to the volume so if it is 10GB then have a 30GB Snapshot - See link below for more information on how Snapshot works and how to determine the size of the Snapshot.

http://www.open-e.com/library/how-to-resources/

2010.09 Description of Open-E Snapshot