If you look at the block, it is also a 9-patch made up of 5 smaller 9-patches and 4 striped blocks.
If your strips are 1 1/2" finished, that would make the whole block 13 1/2" finished (each portion of the 9 patch finishing at 4 1/2 inches) . That is a good size. If the strips are 2" finished, that would make an 18" finished block. I think it really depends what works best with the size of the quilt you want to make and how wide you want your sashing to be. It is a beautiful pattern.