I am backing up ~800GB from a server to a NAS via Gigabit (destination is a share mounted on the server I need to backup). What value would you recommend for volsize?
With such a large data set and a fast gigabit (which I assume is reliable), I would go for a larger volume size than the default of 25 MB. I would at least say 250 MB or maybe 700 MB (size of a CDROM, if you for some reason or other need to transfer backup to an optical medium).
In the past, duplicity had a bug that caused it to use large amounts of memory if you used large volumes, because it allocated buffers that increased along with the size of the volumes. This issue has now been fixed and you should be able to use larger volumes without problems.
I use 700 mb volumes myself for a ~3 TB backup set.