Just as a reference for future needs. To resize a zvol exported as an iSCSI LUN to a Windows OS, you need to change the zvol size, then change the “iSCSI backend” the LUN changed size and then do a disk rescan on your Windows initiator. Commands are:

  1. zfs set volsize=1T tank/volume
  2. sbdadm modify-lu -s 1T <GUID of the volume>
  3. Then on Windows, go to Server manager, choose Storage, right click on Disk management and select rescan. You should see some allocated space on the disk.

Source: https://www.growse.com/news/comments/growing-an-ntfs-iscsi-volume-that-s-hosted-on-zfs/