Q: What is the best, safe way to enlarge a SoftwareRAID array?
A: The best, safe way is the hard way:
We do not support of other possible, less than safe options todo this task, for that its stripped away from here.
Please note that if you have a ZFS mirror (RAID1), you can replace one disk at a time with bigger disks, thereby enlarging the array, sometimes without even turning off the system if all your hardware is hot-plug compatible.