Well, it's really up to you but let me put it this way ...
In order to give you a zpool of your own we need to give you a full blown VM (bhyve) with resource guarantees and your own IP address, etc.
So there is a 1TB minimum and no discounts.
Alternatively, if you just get a plain old rsync.net account the minimum account size is much smaller and if you're an expert and don't need any (borg specific) support there is a discounted plan[1].