File size is always shown in kB, MB, GB, etc
If we set Thunar to show files size in KiB, MiB, GiB, xfdesktop won't change, so the system will be inconsistent. I suggest xfdesktop to either detect Thunar setting or to create an option in Desktop Settings dialog to change how file sizes are shown.
Also, while Thunar shows KiB (uppercase 'k'), xfdesktop shows kB (lower case 'k'), which is another inconsistency.
Version tested: 4.17.1