As far as I understand, ZFS doesn't work at all with disks of differing sizes (in the same array). So if you try it, it just finds the size of the smallest disk, and uses that for all disks. So if you put an 8TB drive in an array with a bunch of 10TB drives, they'll all be treated as 8TB drives, and the extra 2TB will be ignored on those disks.

However, if you replace the smallest disk with a new, larger drive, and resilver, then it'll now use the new smallest disk as the baseline, and use that extra space on the other drives.

(Someone please correct me if I'm wrong.)