Like Grammahunt said you could just decide what size you need. Figure the size of the borders and subtract from final size. each block is finished at 6 inches and the alternating shashing with the four patch is finished at 4 inches wide.
15 of the 6" blocks would equal 90" plus 6" border that would make it 102" wide.
It seems as the center block is 6" plus the 4" shashing with four patch would be 10" decide how many rows you need to equal your length.
Good luck, it looks like fun