Increase the ZVOL extent, then in Windows use the Disk Management console to first "rescan disk" and then "extend volume." Should be all set then.
If the ZVOL doesn't show up with the new size, edit it again, change the contents of "comment" field, and save. Not sure if this is still needed, but I had issues with the FreeNAS iSCSI target (ctl) not re-scanning the ctl.conf file (with the new size) and changing the comment field was enough to trigger that.
edit: I see you're using file-based extents and your size is in bytes. You can't do anything about the former without doing a send/recv to a zvol based extent, but the latter you can use the G/T suffix to simplify the display. eg: you're currently at "500G" and you can just write in "600G" or "1T" to expand to that size.