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.
- zfs set volsize=1T tank/volume
- sbdadm modify-lu -s 1T <GUID of the volume>
- 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.