IDWriteRemoteFontFileStream::GetLocalFileSize method (dwrite_3.h)
GetLocalFileSize returns the number of bytes of the font file that are currently local, which should always be less than or equal to the full file size returned by GetFileSize. If the locality is remote, the return value is zero. If the file is fully local, the return value must be the same as GetFileSize.
HRESULT GetLocalFileSize( [out] UINT64 *localFileSize );
Receives the local size of the file.
This method returns an HRESULT success or error code.