I'm trying to create a partition of 20GB size exFAT filesystem, but no partitioning software allows me (even Windows Disk Management). With 40GB it allows me, so I presume there is a minimum size for an exFAT partition. Somebody can reference any documentation explaining more about this?
On Windows 10, there was no problem formatting a 16 GB flash drive as exFAT. Admittedly, this is not a direct answer to your question, but at least it narrows the issue to either HDD vs. flash USB drive, or perhaps Windows version (which you did not state).
You might try to format using another version of Windows. exFAT is an MS proprietary format, so third-party tools might not work as well. Have you tried the tools at AlternativeTo?