Is 8" the finished size of your block? If so you just need to multiply the finished size by the number of blocks across & down.
So 7 x 8 = 56. & 9 x 8=72
Then you can decide how wide you want the borders. Sorry I don't know the size of a twin bed. I always measure anyway to make sure I allow enough for the drop on the sides & bottom.
Good luck.