In short - the Snapshot should be 3 times the amount of changes you anticipate to accrue with the volume you are writing to, so if you have a 1TB volume and you expect to have 50GB for that day then a 150GB Snapshot volume will be best. Please find the following links that will help you to know more about Snapshot.

http://kb.open-e.com/Snapshot_159.html

http://kb.open-e.com/How-do-snapshot...cation_49.html

http://kb.open-e.com/How-do-snapshot...-task_102.html

http://www.open-e.com/service-and-su...-to-resources/
Open-E Snapshots:
2010.12 Description of Open-E Snapshot