How to determine the length of the RBF file

The length of the file is found within the file but it's encoded. The best place I can point you too is the assembly source code for the NIOS II bootloader found in "c:\altera\90\nios2eds\components\altera_nios2\boot_loader_sources\boot_loader_epcs_bits_sii_siii_ciii.S":

    //
    // If we arrive at this point we have verified that there is valid-looking
    // configuration data. Extract its length.
    //
    // Like searching for the Software-Device ID code, the configuration
    // bitstream length is encoded in a particular bit of a run of bytes.
	// Total length field is 32 bits, which goes from CB Option bits 86..55
	// corresponding to bit of  byte to byte (for the most 
	// significant 7 bits) followed by the range of bits
    // made up of bit of bytes. The above note about EPCS
    // bit reversal still applies (so we're really looking at bits and ,
    // respectively.
    //