First of all, you may refer to the user guide below for the maximum bitstream for particular devices:
Say you are choosing Stratix 10 GX10M, the flash memory that you are choosing should be higher than 654 Mbits.
For configuration devices, you may refer to link below for recommended configuration devices:
Hi Murali kumar M,
Yes, for the firmware file (.elf), it is not part of the bitstream. You will need to program that after bitstream is downloaded.
Thanks and regards