Format is done after the device is part of the array, so parity can be updated.
Default Unraid xfs format is:
mkfs.xfs -m crc=1,finobt=1 -f /dev/mdX
where X is the disk #, since I don't use xfs for a long time not current on the options, but probably crc=0 would get rid of the extra space, google "mkfs.xfs man page" for the options.
Format will likely not work on a mounted filesystem, you'd need to manually unmount, probably easier for you to do this:
-stop array
-click on the disk you want to format and change it to a different filesystem
-start array, that disk won't mount
-type the correct mkfs command for the disk on the console (all data on that disk will be deleted)
-stop array, change its fs back to xfs/auto
-start array, if you used the correct format options it will now use less space.