[FreeRTOS+FAT Standard API Reference]
size_t ff_filelength( FF_FILE *pxStream );
Return the length in bytes of a file that has been opened for reading.
A pointer to the file being queried. This will be
the same pointer returned by ff_fopen() when the
file was originally opened.
If the length of the file was successfully obtained then the file’s length is returned.
If the length of the file could not be obtained then 0 is returned and the task’s errno is set to indicate the reason. The length of a file can only be obtained if the file is open for reading and the file’s length fits in a variable of type size_t.