I was interested to find out how much storage is needed to store the bitstream of a minimal design for a Spartan6 FPGA. The minimal design chosen is just a straight-forward 8-bit binary counter. The results are:
LX25 Raw/Compressed: 801556/233934 LX16 Raw/Compressed: 464290/134942 LX9 Raw/Compressed: 340697/118773
The sizes stayed the same with and without CRC checking. Looks like the memory storage needed is between 8Mbits (raw) and 1Mbits (compressed).