Answering to your last sentence and not the title question, since this seems to be the reason for it:
For zfs, the drives need to have the same size in an array for a meaningful combination. The size of the zpool is limited by the smallest disk in the array for raidzX modes.
If you want to enlarge a pool, you need to make sure that you replace your disks by larger disks one-by-one and let the pool resilver between each step. After you added the last disk, the pool automagically enlarges itself to the new maximum capacity. Sometimes, a export and reimport of the pool with
zpool export tank followed by
zpool import tank is needed to trigger the process.
I have done this several times now over the years, and it was always a sufficient solution. If you plan your pool from the start and max out your slots, the growth option available is similar to other filesystems, since you always need to replace disks.