I have answered your questions below.
"Any idea what is the optimum size for minimum write operation?"
ANSWER: You may have to create some test Snapshot volumes and experiment with the reserved capacities to find the best capacity for your environment, but I did find this general recommendation in our Snapshot documentation.
"The default setting for reserved capacity is 40% of the capacity of the base volume, and usually this capacity is sufficient."
"Why SS volume is not utilizing snapshot reserved space?"
ANSWER: Your question is not explicitly answered in the documentation, but this is my theory.
The Snapshot group reserved capacity stores changes made to a base volume. As scheduled Snapshots are taken, the old data that was over written in the base volume is stored to the Snapshot group reserved capacity. If the Snapshot group reserved capacity becomes full, then the oldest Snapshot images will be purged or write requests to the base will be rejected (depending on the configuration).
If a Snapshot volume used the Snapshot group reserved capacity also, then snapshot images, schedules or host writes to the base volume could be disrupted if a Snapshot volume had an unexpected increase in host writes.
Because the Snapshot volume has its own Snapshot volume reserved capacity, we do not have to worry about the Snapshot volume's usage interfering with existing snapshot images or blocking host writes to the base volume.
"Will copying from that SS volume require write space as well, even so data was copied to another volume?"
ANSWER: If you simply want to copy data from a Snapshot volume without writing to it, the copy operation only requires read access to the Snapshot volume, and read-only Snapshot volumes do not need the Snapshot volume reserved capacity. Snapshot volume reserved capacity is only needed when a Snapshot volume has write access.
In case you have not see this guide already, I recommend reading our SANtricity Snapshot feature Overview and deployment guide to learn more.