You can increase any of the Logical Volumes including the Snapshot, just go where you created the NAS or iSCSI Logical Volume and select "Modify" for that Logical Volume. We do not have a way to shrink a Logical Volume. Link below has allot of details on how our snapshot works.

http://www.open-e.com/library/how-to-resources/
Open-E Snapshots
# 2010.12 Description of Open-E Snapshot

http://www.open-e.com/library/webcasts-and-videos/
2010.12 How to work with Snapshots and how to replicate your data