Rakesh,
It really is less about operating system and more about protocol.
Both Snap Creator and Domino really don't care much about the host OS.
Snap Creator 3.6 can do all backup and restore operations with the Domino plug-in through the NFS protocol.
However, when using block based protocol (FC or iSCSI) the Domino plug-in can only perform single file restore operations through the CLI and it is necessary to mount/unmount a snapshot or clone outside of Snap Creator.
So, to your above:
a) Doesn't matter, but NFS protocol offers the most full features in the Snap Creator GUI.
If using NFS no mounting is needed, but if using iSCSI or FC you will need to manually mount a snapshot.
If you have the option, then this really depends what you are more comfortable with OS wise
2) Once the agent is installed the backup and restore operations can be managed through the GUI. If using iSCSI/FC you may need to mount/unmount a snapshot and run Snap Creator through the CLI, but you can do this on a different Domino server if that is an option for your environment.
3) Sizing is managed much like the sizing for the Domino transaction log volume - you will need to monitor for the first few days.
It is very difficult to suggest sizing as each environment is different and it depends on the utilization of this particular server.
If you are not using archive logging this can be a fairly small volume as we are only storing the data that changes between quiesce/unquiesce - typically a few kb per database. If you are using archive style transactional logging you wil also be storing .txn files in the change info volume to allow restores during your retention period.
4) restore needs to be performed on the same volume where the Domino data is located - most customers typically restore to a subdirectory of the Domino data directory so they can share a link to the restored file to the end user.
Feel free to let me know if you have additional questions.
Thanks,
John