Assuming an uncompressed configuration, you can find the amount of space used for the configuration in the documentation for the FPGA you are using. You could also generate an RBF file and check the size of that.
If using compression, things seem more complicated. It seems to depend how you generate the file as to what size it will be. For example, I get different size files if I generate a compressed RBF file versus running sof2flash. Of course, if using compression, the size will also vary depending on the design contents.
