Steps are mostly as follows

1. shutdown the vm & snapshot or back up image
2. resize the disk image. NOTE: must be larger to retain contents. Use one of the following, depending on where the disk image is stored
   1. truncate -s 10G target.img # local image as a file
   2. lvextend -L 10G /dev/vgname/lvname # an LVM volume
   3. zfs set quota=10G storage/target # a ZFS volume
3. boot up resized opnsense
4. Find

   gpart show               #Show all to select the correct target drive
   gpart show vtbd0         #Will likely show a corrupted GPT ....
   gpart recover vtbd0      #Repair the "corrupted" GPT
   gpart show vtbd0         #Show again to verify and find the partition we're interested in

5. This will show something similar to the following

   =>      40  20971440  vtbd0  GPT  (10G)
           40    532480      1  efi  (260M)
       532520      1024      2  freebsd-boot  (512K)
       533544  20437936      3  freebsd-ufs  (9.7G)

6. Now, resize the partition

   gpart resize -i 3 vtdb0  #Resize the freebsd-ufs partition (free space MUST be below the partition)
   gpart show vtbd0         #Verify resize is what we expect
   growfs /dev/gpt/rootfs   #Grow the filesystem
   fsck -y /dev/gpt/rootfs  #Force a filesystem check/repair

7. df should now show expanded file space – reboot for good measure

• https://unixcop.com/how-to-resize-and-growing-disks-in-freebsd/