FreeBSD defaults to 512k sectors, and there’s no provision in ZFS on FreeBSD to change this. If you try the following that works on Solaris and ZFSonLinux, you’ll be told the option doesn’t exist:

# echo "Does not work on FreeBSD"
# zpool create - 

But you can use:

# gnop create -S 4096 /dev/diskid/DISK-xxxxxxxxxxxxxx

Once you’ve done that, create your zpool on it/them and export:

# zpool create [..] tank mirror \
    /dev/diskid/DISK-xxxxxxxxxxxxxx.nop \
    /dev/diskid/DISK-yyyyyyyyyyyyyy.nop
# zpool export tank

Delete the providers, then re-import the pool.

# gnop destroy /dev/diskid/DISK-xxxxxxxxxxxxxx.nop
# gnop destroy /dev/diskid/DISK-yyyyyyyyyyyyyy.nop
# zpool import tank

Now you have the correct sector size.