I'd like an option to display IEC sizes (KiB, MiB, GiB etc.) instead of SI sizes. 

Currently Thunar uses gLib g_format_size(). There is also g_format_size_full() that allows a choice between IEC and SI.

IEC sizes are familiar to a lot of users and useful in cases such as working with FAT filesystems that have 2GiB or 4GiB file size limits. Putting a 4.3GB file on a FAT32 filesystem can be confusing.