I don't think it will alter the size, at least it won't according to the layout picture you posted. The only issue I see is if your blocks were supposed to be 9" instead of 12 but that's not a problem either because you can just cut down everything (if you haven't sewn the patches into blocks yet.)
Your sashing looks to be 3" if the blocks are 12"--2" if they are 9" blocks. The sashing will be easier than the Half Square Triangles.