You can use different sized disks, but RAID-Z will truncate the space it uses to the lowest common denominator. If you increase the lowest common denominator, RAID-Z should auto-expand to use the additional space. All parity RAID technologies truncate members to the lowest common denominator, rather than just ZFS.
That's not entirely true, Unraid has mechanisms for unbalanced disks, but they come at a high cost in terms of usability by standard workloads.
Unraid is not a RAID technology:
> Unraid saves data to individual drives rather than spreading single files out over multiple drives
https://en.wikipedia.org/wiki/Unraid#Software-defined_NAS
At least, it is not one in the sense of the original RAID paper that coined the term:
https://web.mit.edu/6.033/2015/wwwdocs/papers/Patterson88.pd...
Is it definitely the LCD? Given drive of size 15 and 20 the LCD would be 1, no? I had assumed it would just use the size of the smallest drive on every drive (so 15+20->15+15=30). When I first read your comment I was thinking of GCF but even that would be fairly inefficient (GCF(15,20) = 5, so 15+20->5+5=10).