Earlier quoted context omitted.
ZFS only needs GBs if you need deduplication. 99% don't, almost everyone is better off with compression. 1GB is the min for ZFS though it can run in a little less.
Too much. I've got a 700-800 MB system. It's unfortunate, as I'd love to have a versioned FS
https://wiki.freebsd.org/ZFSTuningGuide
I've no idea what compromises that'll bring you, or how it would react to you pointing it at a multi TB space.